The Windows codec, CoreAVC, is a video decoder which was developed by a company called CoreCodec, especially for decoding the MPEG-4 AVC (or H.264) video format. It is thought to be amongst the fastest software decoders in existence, rivaling even hardware decoders.
There are only currently two versions of the codec available, both for Windows, one which costs US$7.95 and the other more advanced version which costs US$14.95.
Since a Linux version was not available, Google had been leading an open source initiative to create patches for the codec, which allowed it to then work on certain Linux apps like mplayer, for instance.
The DMCA was invoked to get Google to take down the CoreAVC-for-Linux project on grounds that it had been "reverse engineering without permission", according to CoreCodec's president and CEO, Dan Marlin. But Marlin hinted that this may be only a temporary measure, as his company is in discussions for legalising a version of the codec for linux users, coming soon. He just wants them to pay for it like Windows users already do.