A video picture is actually a continuous series of frames. A pixel requires two bytes of memory to record its value. To transport so much information through the limited pipeline of a computer or network's bandwidth, we have to compress it first. Simply stated, compression works by eliminating redundant information in an image, reducing the amount of data presented at one time. For example, if you're watching a football game on video, the field is green, and it stays green from one frame to the next.
Compression allows the field to be sampled less often to produce the contiguous green color.
Less sampling uses less memory. MPEG is the most popular form of compression. Both work well with motion video. It's these image fields, strung together, that create the illusion of movement. MPEG works by determining what has changed between the two fields and adding only the new information to display the next frame.