H.264 is a video compression technology. It may or may not have been included with the video-player software on your computer. If you have downloaded a video file where you can only hear the audio, chances are good that you have an H.264-encoded file, but not the H.264 codec needed to play it. It may also be a DIVX file or something else, so be sure to check our Fix Movies page if needed.
VLC, the free media player, can play H.264 files on the Mac and Windows. Also for Windows is the free K-Lite Code Pack which will install the H.264 codec and many other codecs that you may need. It also comes with a free media player that we like better than the annoying Windows Media Player.
H.264-encoded files may have different file extensions, such as .AVI, etc. So, you can’t tell it is an H.264-encoded file just by looking at the filename. To find out what codec was used on a file, use the free GSpot program linked-to on our Fix Movies page.