http://www.usacycling.org/news/user/story.php?id=580

citizen< cat5 < Cat4 < cat3 < Cat 2 < Cat1 < Pro

Everyone starts out as a citizen or cat 5. You then complete 10 races or, if you're amazing and win your first 3 cat 5 races (just an example, it varies on what the upgrade coordinator will allow), you can apply for an upgrade and move up to Cat 4. Then, you have to move up from there based upon points, which are awarded based upon your finishes in races. The link above shows how that works.

Races are usually Citizen/Cat 5, Cat4/5, Cat3, and then Pro/1/2, or if its a huge race, cat 1/2 and Pro.