http://electronics.howstuffworks.com/itunes.htm
iTunes compatibility in the realm of portable players is a bit of a quagmire, which is part of the reason why some people just avoid iTunes entirely. You can use the iTunes Mac software with, say, a Creative Nomad MP3 player (see iTunes for Mac OS X: Compatible Players for a complete list). But iTunes for Windows only supports the iPod -- if you connect a Creative Nomad to a Windows machine running iTunes, the software won't see it. There is no version of iTunes for Linux machines. But there are ways around that -- just do a Google search for "iPod Linux hacks."
So iTunes (or at least the Mac version) does support other players besides the iPod. But here is gets even trickier: No matter what computer you use, you can't download (or stream) music you bought at the iTunes Music Store to a non-iTunes player. Music you download from the iTunes Store is protected by the Apple DRM (digital rights management) format, which is a proprietary, protected AAC file format that Apple doesn't license to anybody. The only devices that can play those files are ones with the ability to decrypt the Apple DRM, which includes your computer running iTunes, an iPod, an iTunes phone and your speakers connected to AirPort Express. (You can't play them on Roku's SoundBridge, because Apple licensed Roku the iTunes software minus the DRM decryption.)
To play iTunes Music Store files on a portable player besides an iPod, you have to first burn them to a CD as MP3 files. The DRM encoding doesn't make it to the CD. You then rip the now-unprotected files back into your iTunes library and download them to the player.
So now we know which devices will and won't work with the iTunes software. Next, we'll find out what you can do with the software if you've got the right hardware (or the wrong hardware with the right hack).
System Requirements
Mac Mac OS X v10.3.9 or later
QuickTime 6.5.2 or later
256 MB RAM
Broadband Internet (to use Music Store)
Windows Windows XP or 2000
500-MHz Pentium class processor or better
QuickTime 7.1.3 (included in download)
256 MB RAM
Broadband Internet (to use Music Store)