Why not go straight to the source -
http://www.uspto.gov/web/offices/tac/doc/basic/
There is a single organization overseeing domain name registrations, although they sub out actual management of different suffix domains, so you are assured that if you own mydomain.com, no one else can use it.
There is one caution though. As a web developer, over the years I've seen lots of clients lose their domain names by various means:
* Forgetting their login info and forgetting to renew the domain
* Unscrupulous domain resellers whose contracts (in the fine print) actually say that THEY and not YOU own the domain name. They use this as leverage to keep you on their hosting servers.
My advice? If you aren't registered through godaddy.com, then transfer from your current registrar to godaddy.com right away. Keep your domain login info in a safe place.