Prior to the Olympics admitting professionals, the US National Team was comprised of Cat-1's (the highest level amateur). There are not enough pro's riding in the US to have pro-only races, so most races have a pro/Cat-1/Cat-2 race category.
One would begin racing by purchasing a USCF license, and having never raced before, be a Cat-5 (formerly know as ‘citizens’ or ‘unlicensed’ racers). If one places in enough races as a Cat-5, you would move up to Cat-4 and on to Cat-3 and so on. In each category, the races get longer and the competition tougher.
At any given race, there is usually a Pro/1/2 field, a 3-4 field (sometimes combined, sometimes not) and a 5 field. Also, there are age-graded and separate female fields. Usually, the lower categories go first and the Pro/1/2’s last. The 5’s might ride 20 miles and the Pro/1/2’s 70 miles or more.