Codec
Definition
"A codec is short for COder/DECoder and is basically a small piece of software that is used to Encode and Decode video. In order to compress a video file you need to download and install the relevant codecs on your system." (http://xylem.aegean.gr/~modestos/mo.blog/)
Examples
"Xvid: This is one of the most used codecs. The Official Page is http://www.xvid.org/. Most people have it installed, so by using this codec it is almost certain that your videos will be playable everywhere. It is supported by Linux, MacOS and Windows.
x264: a free software library for encoding H.264/MPEG-4 AVC video streams. As of May 2007, x264 is one of the most advanced publicly available AVC encoders. It is supported by Linux, MacOS and Windows." (http://xylem.aegean.gr/~modestos/mo.blog/)