AVI

AVI stands for Audio Video Interleave, which is a file format (or "container") for audio and video information which is "interleaved". This is a fancy way of saying it's a movie with sound.

Video is stored in "frames", which are snapshot images of a moving picture. These are typically captured (and shown) at a fixed rate, for example, 24 frames per second (fps). Television in the US is 30 frames per second.

Old-fashioned movies, such as World War I newsreels, were shot at a slower frame rate (such as 16 frames per second). When these are added into a modern 24 fps movie, often they are played at the same frame rate, which speeds everything up noticeably. It is possible, of course, to program a computer to play frames at a variable rate, so that the old 16 fps footage would appear at normal speed, and even 24 fps movies could play at 24 frames per second, but to save money or development time, this is rarely done. It seems to be easier to convert 24 fps to 30 fps by playing one frame twice and and the next frame 3 times.

See Video for Windows.