Originally Posted by ericm979

The "standard" BB is BSC. BB90 is another standard where the bearing are a press fit into the frame. BSC (british thread) is a threaded BB, which external cups like GXP will thread into.

The press fit BBs are slightly lighter. Its unlikely that you'll feel much of a difference (assuming equally stiff cranks) and even then it won't make any measureable difference in your performance.