The video multiplexer structures the compressed data into a hierarchical bitstream that can be universally interpreted.
The hierarchy has four layers :
This is the highest syntactic structure of the coded bitstream. It can be looked at as a random access unit.
This is optional in MPEG II. This corresponds to a series of pictures. The first picture in the coded bitstream has to be an I picture. Group of pictures does assist random access. They can also be used at scenes cuts or other cases where motion compensation is ineffective. Applications requiring random access, fast-forwarder fast-reverse playback may use relatively short group of pictures.
This would correspond to one picture in the video sequence. For field pictures in interlaced sequences, the interlaced picture will be represented by two separate pictures in the coded stream. They will be encoded in the same order that shall occur at the output of the decoder.
This corresponds to a group of Macroblocks. The actual number of Macroblocks within a slice is not subject to standardization. Slices do not have to cover the whole picture. Its a requirement that if the picture was used subsequently for predictions, then predictions shall only be made from those regions of the picture that were enclosed in slices.