Perian is the Swiss army knife for Quicktime, to play just about any video format that Quicktime does not support (except WMV) It has been a feature of every Mac I have owned since I don’t remember when.
it’s about to be EOL – end of Life.
The Perian dev team announced that the software will no longer be updated. They will release all the source code to Google Code or Github, but the team is moving on.
More information on the announcement from Perian dev team is on the TUAW site
Alternatives are: VLC and NicePlayer for Mac
Perian is explained as part of this video: