(*Actually, .tv is a special case. Two-letter suffixes normally are only for country codes (like .us, .uk, .jp, etc.). So .tv is actually for the country Tuvalu, founded in 1975. Never heard of it? It is a small former British colony located in the South Pacific Ocean, current population just under 12,000. In 2000, Tuvalu leased its Internet domain .tv for $50 million in royalties over a twelve year period.)
Why so many? It is like why the greater Houston area now has three area codes (713, 281, & 832). There just were not enough .com domain names to go around anymore. Plus, some people wanted to stand out in a crowd. If you have a domain name that ends in .travel or .jobs, people immediately know what kind of site you might have.
So who decides what domain names are allowed to be used on the Internet? The Internet Corporation For Assigned Names and Numbers (ICANN), an internationally organized, non-profit corporation, that's who. "As a private-public partnership, ICANN is dedicated to preserving the operational stability of the Internet; to promoting competition; to achieving broad representation of global Internet communities; and to developing policy appropriate to its mission through bottom-up, consensus-based processes."
Wondering if a particular domain name might be available? Search WHOIS. Note that anyone may register a domain name ending in .com, .info, .net, and .org. All of the other domain name suffixes have various restrictions (such as only businesses may use .biz, only sites for individuals' personal names may use .name, etc.). WHOIS will tell you to whom a domain name is registered, when that registration expires, and how to contact the registrant.
Domain names like hcpl.net (HCPL) or houstonlibrary.org (Houston Public Library) or hctx.net (Harris County) or houstontx.gov (Houston) make it easy to quickly give a Web address, such as over the radio or TV or when speaking to a friend. This is crucial when marketing your site to your potential customers. Who wants to read out a long string of numbers (what computers actually use after the domain name is translated into its corresponding IP numerical address) and expect anyone listening on the radio in their car while driving to remember the address for that site!
For more information see: