A codec (COder/DECoder) allows one to encode/decode audio and/or video. If you have video and you are getting a 'black screen' but you still hear sound, you would need the correct video codec to decode the video. The same can be said for being able to view the video while not being able to get the sound.

Most A/V files on a PC have the infamous .AVI extension and there is a lot of confusion surrounding the extension.

AVI (audio Video Interlace) is simply a file 'container' for holding together the 'video stream, 'audio stream' and file header. In order to recode video or audio is is important to be able to isolate the video and audio codecs that were used to create the file.

One handy utility for doing this is a small application called Gspot. Once Gspot determines what codecs are used in the streams it will inform you what they are called (by a FourCC code) and if they are on your system.