H.264/MPEG-4 Part 10 or AVC (Advanced Video Coding) is a standard for video compression. The final drafting work on the first version of the standard was completed in May 2003. H.264/MPEG-4 AVC is a block-oriented motion-compensation-based codec standard developed by the ITU-T Video Coding Experts Group (VCEG) together with the ISO/IEC Moving Picture Experts Group (MPEG). It was the product of a partnership effort known as the Joint Video Team (JVT). The ITU-T H.264 standard and the ISO/IEC MPEG-4 AVC standard (formally, ISO/IEC 14496-10 - MPEG-4 Part 10, Advanced Video Coding) are jointly maintained so that they have identical technical content. H.264 is used in such applications as Blu-ray Disc, videos from YouTube and the iTunes Store, DVB broadcast, direct-broadcast satellite television service, cable television services, and real-time videoconferencing.

x264 is a free software library and application for encoding video streams into the H.264/MPEG-4 AVC format, and is released under the terms of the GNU GPL. The code is written from scratch by Laurent Aimar, Loren Merritt, Eric Petit (OS X), Min Chen (vfw/asm), Justin Clay (vfw), Mans Rullgard, Radek Czyz, Christian Heine (asm), Alex Izvorski, Alex Wright and Jason Garrett-Glaser.
It provides best-in-class performance, compression, and features. Performance-wise, x264 can encode 4 or more 1080p streams in realtime on a single consumer-level computer. Quality-wise, x264 has the world's most advanced psychovisual optimizations and has won many awards, . Feature-wise, x264 supports features necessary for many different applications, such as television broadcast, Blu-ray, low-latency video applications, and web video. x264 forms the core of many web video services, such as Youtube, Facebook, Vimeo, and Hulu. It is also used in television broadcast by companies such as Avail Media.

Refrenced from http://www.videolan.org/developers/x264.html