AVI stands for Audio Video Interleave. This is a container video format that specifies certain structure how the audio and video streams should be stored within the file. AVI itself doesn't specify how it should be encoded (just like the streaming format ASF), so the audio/video can be stored in very various ways. Most commonly used video codecs that use AVI structure are M-JPEG and DivX ;-). AVI contains code called FourCC which tells what codec it is encoded with.