Perian is a software app that makes viewing your media files easier. Basically, Perian enables QuickTime application support for a wide variety of media files. Perian is available from several sources, but not the App Store when we looked. It installs easily as long as you have no software already installed that can conflict with Perian.
Perian supports a variety of file types for both video and audio files. All the standard file formats are supported, but there's a ton of less-commonly found file formats supported by Perian, too. Perian works without you realizing it is there. When QuickTime is used to play a file, that file just works (no more error messages). Files that don't normally work on the Mac all work using QuickTime, with Perian handling the file formats.
It's important to note that Perian does not work with QuickTime 10 or above, and the development team for Perian will cease updates in the near future to move to another project. Still, if you use earlier versions of QuickTime and run into file format issues (especially on migrated files), then Perian is a must-have. Its support for a huge number of file formats makes this a useful utility for most people.