According to WikiPedia, "A codec is a device or program capable of performing encoding and decoding on a digital data stream or signal."  In plain English I'd put it this way: a codec is a particular way of compressing audio and video files to save space.

The best known example of a codec is MP3. It compresses bulky audio files such as WAV to much smaller MP3 files.

All codecs involve a trade off between the amount of compression and the resultant quality. If you compress too much the quality loss may become intolerable.  Information scientists are always coming up with smarter ways to get more compression with less quality loss. That's one of the reasons why there are so many codecs and it's also why newer ones keep appearing on the internet.

A brand new PC will be capable of playing certain kinds of media. By default it will be able to play audio CD's, WAV files, certain kinds of AVI files, and Windows WMV files. However, there are a lot of other file formats which it will not be able to play, so additional codecs are needed for the computer to be able to decode those files and play them.

In fact, most computer users have encountered the situation of trying to play a video file only to be frustrated to find they don't have the codec. As a result the video won't play.

While you can install individual codecs in order to play a particular media file, it's more common to install a whole collection; a codec pack. The idea is to give end users a "one stop" installation of a lot of codecs so they don't have to hunt down particular codecs when they come across a file they cannot play.

Typically this involves the use of codecs that are altered versions of existing codecs developed by other companies. Often this is done to avoid copyright issues, but it's sometimes done to extend functionality so the PC can play more file types. However, since almost all codecs are released under a license agreement which prohibit alteration, these altered versions are still likely to be illegal.

And it's not only the law you have to be concerned about. Using an altered codec may cause problems on your PC. It all depends on how professionally the alterations were made. Unfortunately, you have no way of knowing this in advance.

The following codecs and codec packs are considered to have legality issues hanging over them:

• DivX codec - This and the many varieties of it are altered versions of the Microsoft MPEG-4 V3 codec. It is considered old by today's standards
• K-Lite codec pack - Contains the DivX codec, so it is therefore questionable.
• Asus Video codec - Another variation of Microsoft's MPEG-4 codec.

So where can you find free, legal codec packs that will let you view the media you need to view?  The following is a list of codec packs that are legal and free to download and install on your PC:

• XP Codec Pack:  http://www.xpcodecpack.com/
  
• FFDShow:  http://sourceforge.net/projects/ffdshow
  
• Combined Community Codec Pack:  http://www.cccp-project.net/

Be aware that you may have an illegal codec pack installed on your PC even if you didn't download it. That's because a number of media players come with codec packs as part of the installation package.  Often these codec packs show up in the Windows Control Panel Add/Remove programs applet, so you can uninstall them there if you wish. If they are not shown in Add/Remove programs, they will still be visible from the Windows Device Manager.

To see the codecs installed on your PC, right click on the My Computer icon, and select Properties/Hardware/Device manager/Sound Video Game Controllers. You can then select either of the Audio Codecs or Video Codecs items. For each items the codecs are listed under the properties tag. If you want to, you can also uninstall any unwanted codecs from the same location.

• "Windows Media Player for Windows XP Supported Codecs":  http://support.microsoft.com/kb/291948
  
• "The default codecs that are included with Windows Media Player 9 and 10 "http://support.microsoft.com/default.aspx?scid=kb;en-us;899113
  
• WikiPedia entry for "Video Codec":  http://en.wikipedia.org/wiki/Video_codec
  
• WikiPedia entry for "Codec":  http://en.wikipedia.org/wiki/Codec
  
•  List of Codecs by FOURCC:  http://www.am-soft.ru/fourcc.html
  
•  WikiPedia entry for "K-Pack Codec Pack":  http://en.wikipedia.org/wiki/K-Lite_Codec_Pack