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. This article focuses on video codecs.
Converting video files
Playing video files
No matter what codec your video file has, it can be played with mplayer.
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.