QuickTime is a proprietary multimedia platform developed and distributed by Apple Inc. It is available for both Mac and Windows (PC) operating systems. It used to be an optional component, but is now a standard part of the Mac OS installation. It is used for just about any multimedia function in most Mac applications but is just usually used to play QuickTime format media on a PC.
Techopedia explains QuickTime
QuickTime may refer to the multimedia platform and it may also refer to the format that was made for the platform with the extensions of .mov, .moov and .qt. The QuickTime format is very versatile and comprehensive; it can contain any kind of continuous data like audio, video, text and even time-based control information.
The QuickTime framework provides the following services:
Transcoding and encoding audio and video files into different formats
Decoding multimedia data or playback. In Mac OS X, the decoded stream is sent to the Quartz Extreme Compositor (OpenGL)
A plug-in architecture that allows third-party codecs like DivX