Digital Rights Management (DRM) files are music or video files that have been encoded so that they will only play on the device onto which they were downloaded, or onto compatible devices that have been authorized. The purpose of DRM encoding is copyright protection — that is, to stop a user from copying and sharing a file -- so that the music companies and musicians don't lose revenue from music sales.
If you are looking through a media server folder but can't find a file in the music or movie menu of your network media player, it may be that it is a DRM file format. If you can find the file but it won't play on your media player even though other files in the music library can play, it also may indicate a DRM--copyright protected--file.
Music and videos downloaded from online stores —such as iTunes and others — may be DRM files. DRM files may be shared between compatible devices. iTunes DRM music can be played on an Apple TV, iPhone, iPad or iPod Touch that is authorized with the same iTunes account.
Typically, computers and devices must be authorized to play purchased DRM files by entering the original purchaser's user name and password.
In 2009, Apple changed its music DRM policy and now offers all of its music without copyright protection. Songs that were purchased and downloaded from the iTunes store prior to 2009 are copyright protected. However, those purchased songs are now available in a user's iTunes in the Cloud. When these songs are downloaded again to a device, the new file is DRM-free. DRM-free songs can be played on any network media player or device that can play the iTunes AAC music file format (.m4a).
Movies and TV shows purchased from the iTunes store are still copyright-protected using Apple's FairPlay DRM. The downloaded movies and videos can be played on authorized Apple devices but cannot otherwise be streamed or shared. The DRM-protected files will either not be listed in their folders on the network media player's menu, or you will receive an error message if you try to play the file.
At first, DRM was a good idea to help protect musicians and movie-makers from piracy, and the threat of losing revenue from the distribution of song copies that weren't bought. But as more media playing devices were created, we now want to be able to turn on a media player on at home, or a smartphone when traveling, and be able to play those songs we bought.