From LQWiki
Codec stands for coder and decoder and is a system that encapsulates both encoding and decoding of information (as opposed to two separate systems). In computing terms, it is a software library or utility that performs the encoding and decoding on raw data, into and out of whatever format the codec supports.
A common example is video codecs, which can both encode the video images into some other form (usually for compression), and playback the encoded video (i.e. decoding back in a video stream).
There are a large number of video formats, and corresponding codecs.
For examples see:
How can I find out the codec?
You can find out the codec of a file by starting konqueror, click on your "home" icon and chose View|View Mode|Info List View. For every file that has a codec, you get shown what codec it is.
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