A codec is a downloadable software encoder or decoder for a particular media format that is geared towards a specific computer platform. Codecs function much like plugins for browsers, and are used by players or authoring software to work with the specific media type. Most codecs that you will come across are decoders that are geared to presenting media content to players.

A codec is a compresser/decompresser, a bit of software or hardware that takes raw video and compresses it, and can take the compressed video and decompress it back to raw video. Codecs exist for all kinds of compressed video, including DV, motion-JPEG, MPEG, Indeo, Cinepak, Sorensen, wavelet, fractal, RealVideo, vXtreme, and many others. (Indeo, Cinepak, Sorensen, RealVideo, and vXtreme are trademarks of their respective trademark holders.)