What Are All Those ITunes Files?
You've probably peeked into your iTunes folder (It's located in your user folder: Music: iTunes) and may have been perplexed by all the files and folders it contains, wondering what they're all for. Here's an overview of the files and folders that iTunes uses, both in the iTunes folder and elsewhere.
To start with, your iTunes folder houses, by default, most of the files that iTunes uses. The basic files are the iTunes Library file, which contains information about all your songs, videos, and audiobooks, as well as your playlists. This is the most important file for iTunes: if it doesn't find a library file, iTunes will ask you to choose a library or create a new one.
You'll also notice an iTunes Library.xml file nested in the same folder. This is just another version of the iTunes Library file, but in a format that is readable by other programs that may be able to access your iTunes content--including Apple's Pages, Keynote, and iMovie, which use a special media browser palette to let you import music into your documents. You may also see this in the open dialog boxes of other programs; a number of programs have a Media section in the sidebar of open dialog boxes that provides the same access as Apple's media browser. While the iTunes Library file is essential, the XML version of the file is actually expendable; if you delete it, iTunes will recreate it for you. If you ever have problems accessing iTunes content from other applications, delete that file; it might be corrupted.
By default, your music is in an iTunes Music folder in your iTunes folder. If you've changed the location of your iTunes Music folder in iTunes' Advanced preferences, you may not have an iTunes Music folder; your folder will be another folder elsewhere on your hard disk, or on another disc, and needn't have the standard name. Also, if you have unchecked Copy Music Files Oo iTunes Music Folder When Adding To Library in the Advanced preferences, some or all of your music may be in other locations. (You can always find out where a specific track is by Control-clicking a track and choosing Show In Finder.)
Starting with iTunes 8, there are also two files with the .itdb extension. iTunes Library Extras.itdb contains some Gracenote CDDB information. iTunes Library Genius.itdb contains information about your music library for iTunes' Genius feature. If you've never turned on Genius, you may not have this file, but if you have and turn Genius off, this file will still be there.
(Image Caption: Your iTunes folder contains lots of different files and folders.) There are also a handful of folders in the iTunes folder, other than your iTunes Music folder. iPod Games stores games that you may have purchased for your iPod (other than the touch or the iPhone), as well as information about these games and those that are included on the iPod. (There's also an iPod Games Library file to keep track of purchased games.) Previous iTunes Libraries contains, well, previous iTunes libraries. When you upgrade iTunes, the program may need to upgrade the format of your library file. When it does, it keeps the old versions just in case. If you have an iPod touch or an iPhone, you'll have a Mobile Applications folder that contains all the apps you've downloaded from the App Store. And you'll have an Album Artwork folder, which contains both album art downloaded from the iTunes Store and a cache of album art that iTunes uses in its various displays--for Cover Flow and regular album art displays in your iTunes library or playlists. (Note that this folder may contain a lot of files: my 38,000-song library has an Album Artwork folder that's more than 800MB in size.)
There's one more location for iTunes files: in your user folder's Library: iTunes folder. This folder contains iPod Software Updates, whose contents may take up a lot of space. It also contains an iTunes Plugins folder; you'll have something here if you've installed any third-party plugins, such as equalizers or visualizers. And if you use AppleScripts with iTunes such as those on Doug Adam's Web site you may have a Scripts folder here as well. If you have this folder, your AppleScripts show up in a Scripts menu in iTunes (it has an AppleScript icon).
For completeness' sake, let me mention a few other files. Your iTunes preferences are stored in your user folder's Library: Preferences folder, named com.apple.iTunes.plist. There are two other iTunes files there as well: com.apple.iTunes.eq.plist records your equalizer settings, and com.apple.iTunesHelper.plist tells whether iTunesHelper, a helper application inside the iTunes package, has been added to your login items. Finally, in that same folder is a file called CD Info.cidb, which records track information for CDs you've had iTunes look up in the Gracenote CD Database. It also saves the state of each CD, such as whether you've joined tracks while ripping (if you insert a CD that you ripped in the past with joined tracks, you'll find that the tracks appear joined again, because the CD info in this file tells iTunes the previous state). Also, you'll find that CDs for which you've retrieved information in the past won't query the CDDB; if you want to force iTunes to look again, you can choose Advanced: Get CD Track Names.
Oh, one more thing... If you have an iPod touch or an iPhone, and have iTunes make backups of the device's data, these backups are stored in your user folder's Library/Application Support/MobileSync folder.
[Senior contributor Kirk McElhearn writes about more than just Macs on his blog Kirkville.]