Mac OS X Tiger/Meet the Applications/QuickTime
Introduction
QuickTime, Apple's media player, is widely considered to be one of the "big three" audio/video players available. The other two, Microsoft's Windows Media Player and Real Media's Real Player, are also available for the Mac, but are not included out of the box.
QuickTime files are actually just digital "shells" containing two different files: an audio file and a video file. This provides content makers with a huge amount of flexibility.