The iTunes Genius feature can help you discover new music you haven’t heard before, but it can also present music you already have in your iTunes library to you in new ways - specifically in the form of Genius playlists.
Genius playlists are different from playlists you create yourself or even smart playlists, which are created based on sorting criteria you determine. Genius playlists use the collective intelligence of the iTunes Store and iTunes users to create playlists that pair related songs together and make playlists that will sound great (or so Apple claims).
Applying this Genius, believe it or not, takes almost no work at all. Here’s how it works.
First, make sure you have iTunes 8 or higher and have Genius turned on. Then find the song you want to use as the basis of the playlist. Once you’ve got it, create the playlist either by clicking the Genius button in the bottom right of the iTunes window (it looks like an atom – a few rings with spheres on it) or right-click the song and choose "start Genius."