A video stream consists of bytes representing pixel values. For color movies, each pixel is typically represented by three bytes. A collection of pixels make up a frame, a still image of the scene at a certain time. A sequence of frames make up the video.
Digital images and video are resource demanding when it comes to storage or transfer requirements. It is thus often necessary to compress the data by finding alternate representations. One may take into account the way the human visual system works, and remove certain information without making the loss too noticeable for human spectators.
Single image compression consists of three steps: Transform, quantizing and coding. The transform, typically DCT, reorganizes the pixel data. The quantizer removes ``unnecessary'' information, while the coding step performs a general compression scheme on the remaining data.
When compressing video, one may take advantage of similarities between nearby frames. With motion compensation, the coder tries to find the most equal block (small sub-image) in an already seen frame, by searching a small neighborhood of the current block. The current block is then coded using the prediction error from the matching block.
Two families of international video compression standards exist: The CCITT family, including H.261 and H.263, and the MPEG family.