Your iTunes library contains a lot of different types of content. The heart of your library is media files: music, movies, TV shows and music videos. But iTunes also stores audiobooks, apps, podcasts, iTunes U content and ringtones.
When you launch iTunes for the first time, four types of content are shown in the iTunes source list; that’s the sidebar on the left side of the application. These are Music, Movies, TV Shows and Radio. If you add other types of content, other elements will display at the top of the source list. For example, add an audiobook and Audiobooks will show up; add a ringtone and Tones will be added to the list. Some of these items will also display numbers, showing the number of unlistened podcasts, the number of TV shows you haven’t watch, etc.
There is one temporary item: Rentals. This will only display as long as you have active rentals in your iTunes library. When rentals expire, this item disappears, along with the files for the rentals.
If you have removed all of a specific type of content, or if you simply don’t want to display it in the iTunes source list, you can go to iTunes’ General preferences and uncheck the items you no longer want to see. For example, if you never listen to Internet radio stations through iTunes, you can uncheck Internet Radio. If you don’t have any more ringtones, uncheck Tones. If you have only music in your iTunes library, uncheck everything but Music.
These options allow you to clean up the iTunes source list and simplify it a bit. You’ll find it easier to move around among your media libraries if you only see the ones that interest you.