Codec: COmpression/DECompression algorithm. It's for reducing file size, as uncompressed audio/video takes a lot of HD space. Generally speaking, if you want to play a compressed video file, you need the same specific codecs as were used to compress it.
Get the latest beta version of GSpot to identify the audio and video codecs in a particular video file:
Codec Sniper can identify and remove broken codecs from your system:
Both are free. Good luck.