To remove Apple's DRM a tool called Requiem is used.
Requiem is a program that removes Apple's DRM (called FairPlay) from songs, videos, and books purchased via iTunes, so they can be played on non-Apple-approved devices like a SlimServer or Linux box. The DRM removal is a lossless process - it is merely decrypting the file, not decoding and reencoding it. Requiem must be run on a computer that is currently authorized to play the DRM'd music.
AppAdvice expects Apple to release an update to iTunes which will block Requiem from working in the near future.