Amazingly GENIUS!

The Genius feature, introduced in iTunes 8, automatically generates a playlist of songs from the user's library which are similar to the selected song. Genius playlists are created by the ratings system and collaborative filtering. An iTunes Store account is required because information about the user's library must first be sent anonymously to Apple's database. Algorithms determine which songs to play based on other users' libraries, and Genius becomes more intelligent given a larger data set. The resulting Genius playlist can contain 25, 50, 75, or 100 songs and can be refreshed for new results or saved. The Genius Sidebar will similarly recommend selections for purchase from the iTunes Store based on the selected library track. Once Genius becomes active in iTunes, it can be used on current generations of the iPod Classic, iPod Nano, iPod Touch or the iPhone. ^[34] iTunes 9 added Genius Mixes, where the Genius software finds similar music and automatically puts them into mixes.^[35]

^{source: wikipedia}