B-Frames - the real meaning

The name is short for Bidirectionally Predicted frames. These frames can refer to other frames that occur both before and after the B-frame.

In other words a B-Frame can say "this frame is the same as the last frame except that the football player has moved, and the ball is from the next frame except that it has moved"

Specific differences

In MPEG-1, 2 and B-frames can refer only to the previous and next Key or P-frame

In h.264. B-Frames can refer to multiple key, P and B frames