An Apple ID is the email address you use as a login for just about everything you do with Apple, including using iCloud to store your content, buying songs from the iTunes Store, and downloading apps from the App Store. An iCloud account, iTunes account and Apple ID are all the same thing. Apple just refers to them a bit inconsistently.
iCloud Account - Typically refers to the Apple ID that is used for iCloud services, such as backup and syncing documents and settings between apps. This is set in Settings → iCloud → Account.
iTunes Account - The Apple ID that you use to purchase downloads from the iTunes, iBooks and App Stores and what syncs your iTunes Match songs, if you use that service. It's set in Settings → iTunes & App Stores → Apple ID.When you set up your iPhone, iPad, or iPod touch, you can use the same Apple ID for iCloud services and purchases on the iTunes Store, App Store, and iBooks Store. You can also use one Apple ID for iCloud services and another Apple ID for store purchases (including iTunes in the Cloud and iTunes Match). You will get all the benefits of iCloud whether you use the same Apple ID for iCloud and store purchases, or different IDs for each.You can use different Apple IDs for iCloud and Store purchases and still get all of the benefits of iCloud. To sign in with your Apple ID on your devices follow the steps below:
On your iPad: In Settings > iTunes and App Stores, sign in with the Apple ID you want to use for Store purchases (including iTunes in the Cloud and iTunes Match). You may need to sign out first to change the Apple ID.
On your PC: Open iTunes. Enter the Apple ID you want to use for Store purchases (including iTunes in the Cloud and iTunes Match) click iTunes Store > Quick Links: Account.
Note: Once a device or computer is associated with your Apple ID for your iTunes Store account, you cannot associate that device or computer with another Apple ID for 90 days. An Apple ID can only be associated with up to 5 PC's at a time in iTunes the program.Information taken from: http://support.apple.com/kb/ht4895