What is the CCCP?
The full title is the Combined Community Codec Pack. The name is also a pun on the Russian name for USSR, Союз Советских Социалистических Республик, abbreviated as "СССР".
The Combined Community Codec Pack is a simple playback pack for Windows with the goal of supporting the majority of video formats in use today. The CCCP attempts to be as unintrusive on your system as possible. Why CCCP? Because it achieves as much as possible with as little as possible. Other codec packs tend to either randomly not work, break the system, or install far too much cruft. With the CCCP, you know that when you install it properly, it just works. You don't need to mess around in the options to get it to work, and chances are it will resolve any other conflicts on install as well. CCCP was invented so you can "Uninstall everything else, install the pack and live happily ever after".
Using CCCP is simple. Uninstall all codec packs and codecs first. Then just download the installer and run it. Leave all options untouched unless you know what you're doing.
The pack is also now officially endorsed by the Matroska Project, the developers who designed the MKV format.