General outline
A video codec is a piece of software (more correctly, an algorithm) that offers a lossy compression for video. Historically, video was stored on tapes or reels. When the compact disc entered the market as the ultimate replacement for vinyl, there was not really a reason why the format could not be used to store video data. After all, if audio could be translated to binary data, why couldn't video?
The downside is that video carries much more information than just sound alone. In order to make it fit on a single disc the data should be compressed.
Audio and video call for a different method of compression than other data. Software programmers and mathematicians have tried a number of solutions for tackling this problem.
When encoding, there is always a balance between quality and quantity - the compression ratio. Video offered on-line has to take the lowest common denominator into account - dial-up. Since the transfer speed is low the signal has to remain intelligible, and therefor quality is sacrificed.
For other media where quality matters more and bandwidth is not an issue (such as DVD) quality is maximized.
Usage
Codecs are in daily life found in DVD (MPEG-2) and VCD (MPEG-1). Online video material is encoded in a variety of codecs, and this has lead to the availability of codec packs - a pre-assembled list of commonly used codecs combined with an installer.
Encoding media by the customers themselves has seen an upsurge with the availability of DVD-writers. Since commercially available DVD's are usually bigger in size (dual-layer) and dual-layer DVD-writers are not as common yet the material has to be compressed again, sacrificing quality so the media will fit on a single disc. While initially an arcane process requiring various tools where output was "chained", software manufacturers have responded with one-click duplication.
Commonly used codecs
Unlike the VHS-BETA and DVD+/- wars, codecs do not commonly war with each other. They all try to make a better product, but in the end one codec is going to replace them all. So you can use a codec with confidence that it will not soon become unsupported or obselete. Here is a guide to the main CODECS used.
MPEG-1: Seen sometimes for Internet downloads but is more commonly used for the Video CD standard. It offers roughly the same or better video quality than VHS but not as good as DVD or SVCD.
MPEG-2: Used on DVD and in another form for SVCD. When used on a standard DVD, it offers great picture quality and supports widescreen. When used on SVCD, it is not as good but is certainly better than VCD. Unfortunetly, SVCD will only fit around 40 minutes of video on a CD, VCD will fit an hour.
WMV (Windows Media Video): Microsoft's proprietary format. It can do anything from low resolution video for dial up internet users to High Definition video to view on an HD TV. files can be burnt to CD and DVD or output to any number of devices. It is also useful for Media Centre PCs.
RealVideo: Developed by Real Networks. A popular codec in the earlier days of internet when bandwidth was scarce. Now less popular because the required player adds all kinds of unnecessary extras, and monitors usage behavior.
QuickTime: Strickly speaking QuickTime is not a codec, but a file format, API set, and media player developed by Apple. It supports many popular codecs. Some of the more popular include Cinepack, Sorenson 3, and MPEG-4 Video. QuickTime also supports a plug-in architecture that allows other popular codecs such as MPEG 2 and DivX to be added by the user.
Sorenson 3: A popular codec used by Apple's QuickTime. Many of the Movie trailers found on the web use this codec.
Cinepack: A very early codec used by Apple's QuickTime.
MPEG4: The latest MPEG codec can be used for internet and on disc like WMV.
DivX & Xvid: Types of MPEG 4.
All of the codecs above have their qualities and drawbacks. Comparisons are usually published every time a new codec is released. Compression ratio and artifacts are usually the deciding factors.
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