Apple ID gives users access to buying (or downloading for free) and later free re-download of many Apple-based resourcesApple App Store Includes:
iTunes Store: music, movies, TV shows, podcasts, audiobooks, mobile phone ringtones.
iBookstore: books (currently only usable on iOS devices).
Newsstand: magazines & newspapers (currently only usable on iOS devices).
App Store: iOS apps.
Mac App Store: Mac OS X apps.
iTunes Store, App Store (for iOS apps), Mac App Store, iBookstore, and Newsstand all make use of Apple ID. To purchase digital media such as movies and music on the iTunes Store or the App Store, an Apple ID is required. A user can use an Apple ID and password to sign into the iTunes Store or App Store to buy content, or authorize items the user has purchased. The ID is the proof of ownership for the content the user has previously downloaded from Apple digital stores.
The Apple ID allows the user to re-download their purchased content for any of their devices. For iTunes on computers, an Apple ID is authorized to copy purchased content on up to 5 computers at a time. Apple has not confirmed exactly how many iOS devices can use the purchased content of one ID.
According to Apple support “Your Apple ID can have up to 10 devices and computers (combined) associated with it.”
Shopping the App Store requires an iTunes Store account. When you create an iTunes Store account, you must provide a payment method for your purchase. The available payment methods vary by country, but typically you can use credit cards, gift cards, store credit, or PayPal.
In countries where the iTunes Store only sells applications, the accepted payment methods are Visa, MasterCard, and American Express. Other payment types such as gift cards, store credit, monthly allowances, and PayPal are not accepted. Depending on your App Store country, prices are listed in either US Dollars or Euros.
The ITunes gift card can be used in the mac app store to buy the app in apple mac app store.