USA Cycling is the national governing body of racing. It is the 800-lb. gorilla of domestic racing. There may exist some smaller, local bodies that do the same (here in Southern California, we also have CBR-California Bicycle Racing). They issue licenses to race.
Daily or Annual License? If you’re not sure you’re going to race more races over the course of the year and are just trying it out, you may want to only get a daily license. These are around $10 and only good for that particular day and race. You will want to check with the race promoter if they offer daily licenses–they are not required to do so. If you expect to do multiple races, you may want to get an annual license for around $65. It’s good for the whole year. When you arrive at the race and check-in, you’ll want to show them a copy.
In USA Cycling, amateur racers are divided into categories, 1, 2, 3, 4 & 5. Adult racers in these categories, aged 19–30, are sometimes called “elite”, or “seniors” (i.e.: “senior 5′s” or “elite 3′s”). All new racers begin as a cat-5 and work their way up the ladder. To move from a 5 to a 4, you need to start 10 mass-start races (time trials do not count). To move from a 4 to a 3, you need to get points by finishing in the top-places in your races (you can also move to cat-3 by starting a certain number of races). To move from a 3 to a 2 or a 2 to a 1, you need to gain points by finishing in the top placings in races. Cat-1′s and 2′s sometimes move on and become professionals.
Junior racers (boys younger than 19 years old) are also in the categories listed above (girls have their own category system, below). Some events have “Junior” race categories separated by age: 10–12, 13–15, 16–18 or whatever. These categories are not separated by the racing categories (above) and a cat-5 junior could find themselves racing against a cat-2 junior. Non-juniors may not race in these events. Junior racers, however, may race in a cat-3 event (or whatever their category is) against older racers. Their only limitation is the gears they use. Juniors progress in the category system just like all other racers: by gaining points for finishing position.
Adult racers who are aged 35 or older are referred to as “master” racers. They have categories attached to them (1–5) as above. Some events have master’s races defined by the age of the participants: 35+ or 45+ for instance. The promoter may also specify the category of these older racers as well: 35+/cat-1 to cat-4, or 40+/3–4. The racer in these events have to meet both criteria to enter.
Female racers also have a category system as well (1–4 only). Men are not allowed to race in women’s events, but, women are allowed to race in men’s events and even down one category. For instance, a cat-2 woman may race in a men’s cat 1, 2 or 3 race.