A domain name is your address on the web similar to the street address of your house. Just as you can pretty much live wherever you want, you can register any domain name you want provided that the name is not already being used by someone else.
While there are no hard and fast rules to choosing a domain name, for maximum effectiveness, I do recommend keeping these tips in mind when buying one.
Namecheap is currently hosting a trivia contest on Twitter. Every hour on the hour they will tweet a trivia question and the first three correct responses will win a credit in their Namecheap account for $9.69, the cost of a new domain or domain name transfer.