iTunes Store & Mac App Store: About your Apple ID and password

Your Apple ID and password can be used to sign in, buy content, and authorize items you've purchased in both the iTunes Store and the Mac App Store.

Your Apple ID must be a valid email address (such as steve@me.com). Your password must be at least eight characters including a number, an uppercase letter, and a lowercase letter. You cannot use spaces, the same character three times in a row, your Apple ID, or a password you've used in the last year. 

You can change your Apple ID, password, or personal information at the My Apple ID page by clicking the "Manage your account" link at any time. Changes you make to your Apple ID account while you're in iTunes or the Mac App Store are also recognized by other applications where you use the same Apple Account (such as the Apple Online Store or iPhoto). You may be asked to verify your information the next time you use your Apple ID to purchase something in another application.

The country of the primary address associated with your Apple ID is used to determine the correct Mac App Store or iTunes Store country for you, when activating your iPhone, and by other applications in which you use the same Apple Account.