A top-level domain (TLD) is the final part of a domain name — the letters that come after the dot. The most famous is .com, but there are others — .net, .org, .tv and .us, for example. But none holds as much allure as the .com extension.
As of May 2008, there were 76,007,285 active and registered .com domain names. This compares to 11,397,594 .net names, 6,772,308 .org names, 5,037,335 .info names, 1,968,760 .biz names, and 1,412,141 .us names.

There are a number of factors that have given the .com domain name its leadership spot among the top-level domains. One issue, of course, is that it was one of the first domain name extensions available (along with .net, .gov and .edu). Other extensions were added later, including .aero, .biz, .coop, .info, .museum, .name, .pro (Nov. 2000), and .mobi (2005). The .tv extension has existed since 1996 as a country code for Tuvalu. In 2000, the country struck a deal to make the extension widely available to people outside of the country, and in 2006, .tv was first widely marketed as an extension for the entertainment industry. So .com had a pretty significant head start.

Further Information : http://www.pcworld.ca/news/column/027f35ce0a01040801a6601c98929f81/pg0.htm