Resource

Video compression picture types (Wikipedia)
http://en.wikipedia.org/wiki/B_frames

Description

There are three types of pictures (or frames) used in video compression: I‑frames, P‑frames, and B‑frames. An I‑frame is an ‘Intra-coded picture’, in effect a fully-specified picture, like a conventional static image file. P‑frames and B‑frames hold only part of the image information, so they need less space to store than an I‑frame, and thus improve video compression rates.

A P‑frame (‘Predicted picture’) holds only the changes in the image from the previous frame. For example, in a scene where a car moves across a stationary background, only the car’s movements need to be encoded. The encoder does not need to store the unchanging background pixels in the P‑frame, thus saving space. P‑frames are also known as delta‑frames. A B‑frame (‘Bi-predictive picture’) saves even more space by using differences between the current frame and both the preceding and following frames to specify its content.