H.264 or MPEG-4 Part 10, Advanced Video Coding is a way to code videos in order to make them smaller.
It can be used for both lossy compression and lossless compression, though it is usually used for lossy compression. H.264 is used in many places including Blu-Ray players, as well as videos streamed through sites like Youtube. It is also used in some TV broadcasts.
H.264 has a successor called HEVC.