The Genius feature of iTunes was introduced in iTunes 8. The main purpose of Genius was to search user’s library and find songs that are similar to a selected song and automatically generate a playlist of these songs.
To use this feature user must have an iTunes Store account because the process to create the playlists require sending information about user’s library to the Apple’s database. This information is sent anonymously. Once the information about the user’s library is available algorithms determine which songs should be added to the playlist.
These algorithms are based on other user’s libraries’ data. The resultant playlist generated by Genius can contain 25, 50, 75, or 100 songs.
With iTunes 9 Genius Mixes was introduced which finds similar music and automatically puts them into mixes. The main difference between Genius and Genius Mixes is that Genius creates playlists based on a single selected song whereas Genius Mixes creates playlists based on the musical genre of the songs.
iTunes also shows In The Store recommendations i.e. the songs in iTunes Store that the user doesn’t own but should own. These recommendations are also based on the findings of iTunes Genius.