At its simplest, QuickTime is software that allows Mac and Windows users to play back audio and video on their computers. But taking a deeper look, QuickTime is many things: a file format, an environment for media authoring and a suite of applications:

QuickTime Player — for playing back audio and video files

QuickTime Pro — for flexible multimedia authoring

Browser plug-ins — for viewing media within a web page

PictureViewer — for working with still images (Windows only; Preview is available on Mac OS X)

QuickTime Streaming Server — Open source streaming server for delivering streaming media files on the Internet in real time (Mac OS X only)

Darwin Streaming Server — for delivering streaming media with Linux, Solaris and Windows

QuickTime Broadcaster — for delivering live events on the Internet (free download; Mac OS X only)

MPEG-2 Playback Component — for playing back MPEG-2 content (requires separate purchase)