Different codecs translate in different ways, so a video file compressed using the Intel Indeo codec will be different from a file compressed using the Windows Media Video codec, for example.
For anyone interested in digital audio or video a basic understanding of audio and video codec's is essential.
This article on the topic of audio and video codecs should help you:
http://www.updatexp.com/audio-video-codec.html
Kind Regards
Marc Liron - Microsoft MVP