iTunes is an application that was introduced by Apple for Mac OS X computers originally in 2001, and has been present on both Macintosh OSX and iOS devices ever since. iTunes allows users to manage, playback, and purchase music, and sync their library between various devices. iTunes is also used to manage applications and books for iOS devices, like the iPhone, iPad, and iPod Touch.
Music can be imported into iTunes either by CD, by purchasing it in the iTunes Store, or by downloading it elsewhere and importing the audio file (like internet MP3 stores, such as Amazon MP3). By default, iTunes can sync with other computers that have it installed, or Apple's iOS devices (such as the iPhone, iPod Touch, and iPad), along with other iPods in Apple's iPod line. The application is available natively for both Mac and PC computers, along with being pre-installed as an application on iOS devices.
Music from iTunes can be synced to other devices not made by Apple by using third-party applications or utilities that have access to your music library on your computer, such as DoubleTwist (an iTunes alternative) or Google Music. Music that was bought via the early iTunes Store may have built-in DRM protection, and is restricted in the way that it doesn't play natively on some devices.
Content from iTunes can also be streamed wirelessly to devices that support Apple's AirPlay technology, such as Apple TV, allowing music and movies that are stored in an iTunes library to be played on a variety of devices.