According to the USDA's Agriculture Marketing web site there are 2 types of beef carcass:
1) heifers, steers, cows
Prime, Choice, Select, Standard and Utility quality grades apply to #1; and Prime, Choice, Select, Commercial, Utility, Cutter and Canner quality grades apply to #2.
The "grades" of quality are based on marbling, color and firmness of the carcass.
Click here for more specific details from the USDA.