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FourCC, a four character code in beginning of media file, mostly associated with AVI, that tells what codec your system should use for decoding the file.
Method was implemented originally by Electronic Arts back in 80s for Amiga's IFF file format which was an universal multimedia file format that supported audio, video and pictures within same wrapper format. Later this was copied to Apple's AIFF format and Microsoft's RIFF format.
For an excellent list of recognized FourCC codes, please visit this website.