A common misconception about domain names is that they all end in .com. Most sites these days do, but it's only one of many available endings. In fact, there are eight different top level domains in what can be considered the "Old TLDs" (as opposed to the new TLDs that were implemented starting in 2005, and the country-code domains).
The Old TLDs:
.com -- for commercial entities
.com is heavily abused by noncommercial users, as discussed to great length below. All the dumb-asses of the world seem to think that all websites should have .com addresses, whether they logically belong there or not.
.org -- for noncommercial entities
Actually, the RFC document defining the TLD meanings says it's for "miscellaneous" organizations that don't fit elsewhere, but since commercial organizations are covered by .com, the implication is that .org entities are noncommercial. .org is the most appropriate domain for both non-profit and not-for-profit organizations; the distinction between these is important to the IRS but not to the domain name system.
Some discussion at the ICANN site in 2005 indicated that there was a proposal to impose enforcement of noncommercial-organization status on .org registrants, but little clarity about just what that would entail -- would official non-profit accreditation with a governmental body be required, or just common-sense examination of the domain's use to see whether it's predominantly noncommercial? Would personal sites, fan sites, and other noncommercial things that don't have an official organization behind them be allowed to keep using .org domains? However, that proposal didn't go anywhere, and instead a recommendation was made by ICANN's domain name supporting organization to make .org a sponsored domain run by a nonprofit organization and marketed specifically to nonprofits, but not to impose any restrictions on either past or future registrants.
Subsequently, ICANN evaluated proposals (both commercial and noncommercial) for the new .org registry, to take over from Verisign when their contract runs out at the end of 2002. Proposals were supposed to be sensitive to the needs of the noncommercial community and are supposed to market .org in a manner encouraging its differentiation from commercial domains and discouraging duplicative or defensive registrations. The winning registry needed to demonstrate that they have experience running a large-scale domain registry, but they were possibly able to get a grant from a $5 million fund being paid by Verisign to ICANN for the express purpose of helping the .org transition.
.net -- for network infrastructure providers
Next to .com, this is the most heavily abused domain, as few current users can remotely claim to being part of the network infrastructure in the manner intended by the creators of the domain name system. It's instead commonly used by people whose desired name is already taken in .com.
.edu -- for educational institutions
.edu is limited to accredited degree-granting institutions. There was some dispute in the past about whether they must be in the United States or not; there's nothing in the relevant RFC that says this, and several foreign universities were given domains in this TLD, but more recently the registry stopped allowing foreign registrations, and that's written in the current registry's policy now. .edu domains actually used to be more loosely granted to anything educational, so a few non-degree-granting educational organizations such as the San Francisco Exploratorium and various consortiums have .edu domains "grandfathered" from an earlier time.
Until recently, .edu was administered by Network Solutions, but it has recently been turned over to an educational consortium, which has loosened some of the rules -- previously, only 4-year degree-granting institutions were allowed (other than the few grandfathered early registrations), but now community colleges are allowed as well. Some balance needs to be reached. If you're too loose in enforcing criteria, then all sorts of abuses occur. If you're too tight, then people ignore that top level domain in favor of others with looser standards, even if they're not really the appropriate one for the type of entity registering.
.gov -- for governmental entities
.gov is limited by the RFC document to U.S. federal government agencies. However, it always had a few state government sites, like Washington, "grandfathered" from before the "federal-only" restriction was added. Actually, it would be more logical for the federal government to register under .fed.us, like all other countries' governments which are under their appropriate country code.
Somewhere around 2005, they started letting state and local governments get .gov addresses again. For a long time, this seemed to be happening "under the table" with nothing in the official registry site mentioning this availability, but in 2002 it was redesigned to indicate that such entities can now register .gov domains, and that there are proposed changes to open things up to even more related categories. They're also giving .gov domains to Native American tribes, of the form tribename-nsn.gov (where NSN stands for Native Sovereign Nation), even though they already have namespace under nsn.us.
.mil -- for military entities
.mil is limited to the U.S. military. This is another domain that might be better off being under .us rather than at the international top level, but a historical anomaly due to the Defense Department's involvement in the creation of the Internet in the first place.
.int -- for international treaty organizations
This is the most tightly controlled international top level domain, and hence the least used. Even the few organizations qualifying for .int domains don't usually make much use of them.
.arpa -- for addressing and routing parameters
Usually, people think there are seven old TLDs (if they remember that .int exists), but there is actually an eighth global TLD. Normal Internet users never have occasion to encounter it, though it's very important to the internal workings of the Internet. Historically, .arpa was originally the temporary TLD in which sites in the old ARPAnet (the predecessor of the Internet, operated by the U.S. Department of Defense's Advanced Research Projects Administration) had their names until they migrated into their proper place in the domain name system (.edu, .mil, .com, etc.). However, one domain within .arpa became a vital part of the infrastructure -- in-addr.arpa -- used by programs on the Internet that must do reverse lookups from IP addresses to their associated domains. IP addresses have subdomains of in-addr.arpa associated with them which in turn resolve to DNS records showing what domain they belong to. Only "techies" need to know about this, as it's all done behind the scenes, invisible to normal users.
This use of .arpa was long regarded as an archaic legacy usage that really ought to be changed -- in fact, when the .int domain was first set up, in addition to international treaty organizations it was also designated as the proper place for Internet infrastructure functions, with in-addr.arpa not being moved to .int simply because that would break all the existing software that expects it to be where it now is. It was expected that future structures of that sort, such as the one being outlined now for the new IPv6 protocol, would be in .int, not .arpa. However, there seems to have been a recent change of heart, and recent standards-track proposals have decided instead to put new DNS structural lookup records in .arpa as well, with the acronym "retrofitted" to now mean Address and Routing Parameters Area. Thus, proposals now exist to create ip6.arpa and e164.arpa to process queries in IPv6 and E.164 protocols respectively. RFC 3172 documents the current status of the .arpa domain.
Uses and Abuses
Political parties and candidates are among the entities that fit best in the .org domain. While some candidates may really be "for sale", they probably don't want this as their public image, so the commercial .com domain doesn't really make much sense for their campaign sites. It's a sign of the dumbing-down of the Internet over the last four years that most of the U.S. Presidential and congressional campaign sites that have popped up for the 2000 races are in .com domains; in the 1996 races (the first to use the Internet in a big way) they usually made correct use of .org.
An unrelated point on campaign sites, as well as any other site for a temporarily-significant thing (e.g., a particular convention or other event): if you are creating a site for such a thing then you should try to "think generic" when registering a domain for it, and try to pick a name that will be usable for future things, and not just be tied to a single occurrence and obsolete after it. For instance, if it's a campaign site for Joe Schmoe, running for some office in 2004, joeschmoe.org or voteforjoe.org are better names than joeschmoe2004.org, because the latter is useful only in 2004, but the former ones can be reused by Joe for any future campaigns he might be running in.
Unfortunately, the use of .com is so entrenched that sometimes, even when somebody specifically requests registration of a .org (or other) domain, they'll wind up with the .com version by mistake!
Even the U.S. government has joined the "domain abuse frenzy" of using clearly inappropriate top-level domains. They already have complete control of the .gov and .mil domains, but that hasn't stopped them from getting a few .com addresses themselves: The U.S. Postal Service used usps.gov properly for years, but later decided to use usps.com as its primary address, and has also registered other domains such as stampsonline.com.
Wouldn't such addresses as stamps.usps.gov, jobs.navy.mil, and go.army.mil have been more logical? These would identify unambiguously that these domains were official sites of the agencies in question, with no chance that they're really unaffiliated sites grabbed by pranksters or scammers, like whitehouse.com, a site completely unconnected with the White House (actually, a porn site, which some might say is related to the White House after the sex scandals there)?
A news story shows one of the problems that came about due to the Navy's illogical domain usage. Apparently, they had a number of .com domains for different recruiting offices, like navydallas.com, etc., and forgot to renew some of them; this resulted in at least one of them getting re-registered by somebody else as a porn site, a big embarrassment to the Navy. This would never happen with .mil domains, which are unavailable for registration by non-military entities. Also, if they used logical subdomains like dallas.navy.mil, they wouldn't have so many different domain registrations to track that they might forget to renew some of them. Using the system properly works better for everybody.
Are Multiple TLDs Too Confusing?
As part of the general dumbing-down of the Internet as it went mainstream and commercial, there seems to be a sizable body of opinion to the effect that having more than one TLD, with different sites potentially found at the same name in a different TLD, is "too confusing" for the user. Some ICANN dispute panels have held such opinions, regarding a domain as cybersquatting by definition if it matched the name of a famous site in a different TLD. However, part of the charm and serendipity that makes the Internet so interesting is the fact that there can be highly diverse things under the same name in different parts of the namespace.
Some corporate types would like to bulldoze over all of this variety in order to make the Internet safe for their marketing schemes. And some are playing into their hands by registering lots of domains that resemble famous addresses and putting up obnoxious sites that pop up lots of annoying windows at people who go there by mistake, creating some public sentiment against "cybersquatters". However, there is plenty of reasonable use of similar names in different TLDs, for different companies and organizations that happen to have similar names or acronyms, as well as by people operating protest or parody sites that have valid fair-use rights to the name. Suppressing all of them would make the Internet a poorer place. Maybe the big corporate types would like the Internet to have the homogeneity of a shopping mall or fast food chain, but is that in the interests of the users?
It's been a contentious issue regarding the new TLDs whether there should be any "reserved names" that are protected from being registered in any new TLD without special authorization. Various entities have proposed all sorts of things, from globally famous trademarks to geographic place names to generic drug names, to be reserved. It's a lesser-known fact that the current TLDs have some reserved names in effect already. They were snuck in recently as part of revisions to the contract between ICANN and Verisign to operate the registry for .com, .net, and .org. Names on the reserve list are barred from registration, but anyone who already has such a name is still allowed to renew it (so long as it's not taken away by the dispute process). Among reserved words are all one and two letter names, the names of other TLDs (current and proposed), and various words and acronyms that relate to ICANN or IANA (the organizations that run the domain name system).
Want to grab an expired name?
Because of the recent boom-and-bust cycle of Internet business ventures, there are a lot of domain names that are expiring now due to non-renewal, because they were registered by speculators who never managed to unload them, or they were registered as part of a failed and bankrupt e-commerce scheme. Some pretty good names are dropping back into the available pool as a result, giving other people a chance to get them.
Unfortunately, there are still enough speculators and opportunists around that they have created problems in the registration system in their attempts to get the expired names the second they become available.
Because of this, ICANN announced a temporary change in policy at Verisign's request, imposing a temporary moratorium on releasing any expired domains until a scheme can be instituted to get them back into the general pool without causing undue strain on the database. However, after a couple of weeks a new announcement indicated that release of expired domains would resume on Aug. 30, 2005, with new restrictions on access to the registry by automated scripts to prevent the congestion that occurred before.
Verisign has long been accused of holding back expired domains so that they can be channeled into its own domain-auction business, but they have always denied this.
Making ".com" Part of your Company Name?
It was trendy for a while for new (and newly renamed) Internet companies to use their domain name (suffix and all) as their legal corporate name. The Internet site is a distinct entity from the company or organization which owns it, and the ".com" (or ".org", ".edu", etc.) address designates the site, not the company. A company going onto the Internet should pick a domain name based in some manner on its name (e.g, IBM has "ibm.com" and Microsoft has "microsoft.com"), but naming the company after the Internet site is "putting the cart before the horse."
Some business-press writers used "dot-coms" as a somewhat sneering reference to more-hype-than-substance Internet companies. Even TV Guide got tired of "dot-com" companies intruding on Super Bowl viewers, making some sneering references to them in their special Super Bowl issue. Somebody in San Francisco conducted an "anti-dot-com" campaign, putting up stickers lampooning "dot-com mania" by "promoting" such ridiculous and nonexistent sites as AnythingIFoundInMyGarageForSale.com. Meanwhile, the Swedish trademark office announced that they were no longer allowing the registration of company names containing domain or URL suffixes or prefixes, like .com, .se (the Swedish country code), or http://www. Those were among the "early adopters" of the anti-dot-com backlash, but now that position has become mainstream, and companies stick domain suffixes on their names at their own peril.