While some rules govern TLDs from different countries, the .com, .net, and .org name spaces are basically under the first come first served basis. The only restrictions for domains are potential copyright infringement, and these are pretty tough to prosecute.

1. .COMmercial, use for business, personal, and general. There are about 10 times more .com's than any other TLD (top level domain). The .com is the hottest domain property on the Internet (ever heard of a .org millionaire?) To date, there are over 22 million .com domains registered!
2. .NETwork was originally intended for companies & organizations that offer services related to the Internet. While this was the original design, the sheer amount of .coms that have been registered has led to a significant spill-over into the .net name space.
3. .ORGanization domains are generally considered appropriate for non-profit and other like organizations. There are about 1/2 .org domains as .net domains, but there are still spill-overs from the .com name space. Often, what will now happen is a client will want to protect a name, and will register all three.