iTunes Store: About Apple ID and Password

Your Apple ID and password are two key parts of your Apple Account. You can use your Apple ID and password in iTunes to sign in to the iTunes Store, buy content, and authorize items you've purchased. If you already have an Apple Account, you can use your existing Apple ID and password to sign in and buy and authorize items you've purchased from the iTunes Store. Your Apple ID must be a valid email address, for example "steve@me.com." Your password must be at least six letters or numbers, and is case sensitive.

You can change your Apple ID, password, or personal information at any time by signing in and clicking View Account at the My Info page (http://myinfo.apple.com). Changes you make to your Apple Account while you're in iTunes are also recognized by other applications where you use the same Apple Account (for example, the online Apple Store, MobileMe, or iPhoto). You may be asked to verify your information the next time you use your Apple Account to purchase something in another application.

You can also update your personal details linked to your Apple ID should they change.  The country of the primary address associated with your Apple ID is used to determine the correct iTunes Store country, when activating your iPhone, and by other applications where you use the same Apple Account.