What is a bidirectionally predictive-coded picture (B-picture)?

What is a bidirectionally predictive-coded picture (B-picture)?

This is an MPEG picture that is coded using motion compensated prediction from past and/or future reference pictures. Motion vectors pointing forwards and backwards are used, and they may point at either I-pictures or P-pictures. The B-pictures provide the highest compression, but demand knowledge of several pictures. Consequently, B-pictures give a higher delay and call for a larger picture memory. B-pictures are never used as a reference in a prediction. When B-pictures are part of a sequence, the pictures are not sent in chronological order owing to the fact that future P-pictures and/or I-pictures are needed (and therefore must be decoded) for the decoding of B-pictures. The P- and I-pictures have to be sent earlier than the actual point of time to which they relate.