iTunes is a media library management and playback application developed and maintained by Apple for OS X and Windows. It is also used to synchronise Apple media devices and manage their contents.
Apple also uses iTunes to offer their music and app store, where digital music can be purchased. iTunes plays DRM enabled music from the music store along with other various media formats.
iTunes is required to activate an iOS device before it can be used. This is slated to change soon in iOS 5, however.