From Wikipedia - formal USDA differences between Prime/Choice/Select Beef:

There are 3 main grades given by the USDA to Beef from the that is considered for consumption (taken from Wikipedia):
U.S. Prime – Highest in quality and intramuscular fat, limited supply. Currently, about 2.9% of carcasses grade as Prime.
U.S. Choice – High quality, widely available in foodservice industry and retail markets. Choice carcasses are 53.7% of the fed cattle total. The difference between Choice and Prime is largely due to the fat content in the beef. Prime typically has a higher fat content (more and well distributed intramuscular “marbling”) than Choice.
U.S. Select (formerly Good) – lowest grade commonly sold at retail, acceptable quality, but is less juicy and tender due to leanness.
All Certified Angus Beef®, by example, is graded Prime or Choice only.