K-Lite Codec Pack contains a rich selection of codecs, ffdshow filters and DirectShow filters to assist with the playing/transcoding of many media formats. Although it may not sound amazing and may sound technical, in fact it's a must-have download for many a user. This is because all the components in this package extend Windows Media Player to be able to support media (both music and video) formats that it wouldn't otherwise be able to play without them. Also, the codecs/filters can be used to encode media into new formats. This is perfect if you are using a movie production application, for example Windows LIve Mobile Maker, and need to save your project. Not only will you be able to import/edit many more formats, you'll be able to export them as a finished project, too.
For advanced users, there is a set of tweaking and benchmarking tools to allow them to have the codecs the way they want them, though novice/home users probably won't need to use these, as the default options are perfectly adequate.