Gratuitously Stolen from Wikipedia H.264/MPEG-4 AVC
H.264, MPEG-4 Part 10, or AVC, for Advanced Video Coding, is a digital video codec standard which is noted for achieving very high data compression. It was written by the ITU-T Video Coding Experts Group (VCEG) together with the ISO/IEC Moving Picture Experts Group (MPEG) as the product of a collective partnership effort known as the Joint Video Team (JVT). The ITU-T H.264 standard and the ISO/IEC MPEG-4 Part 10 standard (formally, ISO/IEC 14496-10) are technically identical. The final drafting work on the first version of the standard was completed in May of 2003.
...The intent of the H.264/AVC project was to create a standard that would be capable of providing good video quality at bit rates that are substantially lower (e.g., half or less) than what previous standards would need (e.g., relative to MPEG-2, H.263, or MPEG-4 Part 2), and to do so without so much of an increase in complexity as to make the design impractical (excessively expensive) to implement. An additional goal was to do this in a flexible way that would allow the standard to be applied to a very wide variety of applications (e.g., for both low and high bit rates, and low and high resolution video) and to work well on a very wide variety of networks and systems (e.g., for broadcast, DVD storage, RTP/IP packet networks, and ITU-T multimedia telephony systems).
The Hauppauge HD-PVR is a well-supported high-definition TV tuner for MythTV that produces H.264 recordings.
Notice that MPEG4 Video used in broadcasting context mostly refers to MPEG 4 Part 10/H.264/AVC. Not to be confused with MPEG 4 Part 2/divx/xvid.