From Paul Gil,
Your Guide to Internet for Beginners.
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A. "Codec" is a technical name for "compression/decompression". It also stands for "compressor/decompressor" and "code/decode". All of these variations mean the same thing: a codec is a computer program that both shrinks large movie files, and makes them playable on your computer. Codec programs are required for your media player to play your downloaded music and movies.

"Why do we need codecs?"

ANS: Because video and music files are large, they become difficult to transfer across the Internet quickly. To help speed up downloads, mathematical "codecs" were built to encode ("shrink") a signal for transmission and then decode it for viewing or editing. Without codecs, downloads would take three to five times longer than they do now.

"Is there only one codec I need?"

ANS: Sadly, there are hundreds of codecs being used on the Internet, and you will need combinations that specifically play your files. There are codecs for audio and video compression, for streaming media over the Internet, videoconferencing, playing mp3's, speech, or screen capture. To make matters more confusing, some people who share their files on the Net choose to use very obscure codecs to shrink their files. This makes it very frustrating for users who download these files, but do not know which codecs to get to play these files. If you are a regular downloader, you will probably need ten to twelve codecs to play your music and movies.

"What are the common codecs people use?"

Some codec examples are MP3, WMA, RealVideo, RealAudio, DivX and XviD. There are many other more obscure codecs.

"Isn't '.AVI' a codec already?"

ANS: AVI is not in itself a codec; it is a common "container format" that many different codecs can use. As there are hundreds of codecs out there are compatible with AVI content, it can get very confusing which codec(s) you will need to play your video files.

"How do I know which codec to download and install?"

ANS: Your Windows Media Player will often try to communicate to you the 4-character code of the specific codec it needs. Note this code, then visit this website http://www.fourcc.org/fcccodec.htm to obtain the missing codec. For a small FAQ section, follow the link on the left called "Sample Code".

"What are the codecs I should download and install?"

ANS: There is no single best answer to this question. There are so many codec choices. The easiest option is to download "codec packs". Codec packs are collections of codecs gathered in single large files. There is much debate over whether it is necessary to get a large group of codec files, but it certainly is the easiest and least-frustrating option for new downloaders. Here are the codec packs we recommend at About.com:

1. XP Codec Pack XP Codec Pack is a sleek, all-in-one, spyware / adware free codec collection that also offers a good, solid Media Player Classic. Currently just under 6MB in size, XP Codec Pack is truly one of the most complete assemblies of codecs needed to play all major audio and video formats.
   
   
2. K-Lite Codec Pack Very user-friendly and well tested, K-Lite Codec Pack is loaded with goodies. It will enable you to play all the popular movie formats. K-Lite comes in 4 flavors: Basic, Standard, Full and Mega. If all you need is to be able to play DivX and XviD formats, Basic will do just fine. Standard pack is probably the most popular - it has everything an average user needs to play the most common file formats. Full pack, designed for power users, has even more codecs plus encoding support.
   
   
3. K-Lite Mega Codec Pack Mega is a very comprehensive bundle...it has everything but a kitchen sink. Mega even contains QuickTime Alternative and Real Alternative.
   
   Special thanks to technical consultant and guest writer, Joanna Gurnitsky, for this article.