Every iTunes Gift Card (US) works in Mac App Store, App Store, iBooks Store and the iTunes Store to top up your iTunes account credits to be used for app or in-app purchases.
So go ahead and send your gaming buddy the latest strategy adventure game or your loved ones that album you have on repeat. Only find the music, movies, TV shows, programs, publications, or games you wish to give on the iTunes Store, iBooks Store, or App Store. Or from your iOS device, click the "share button" near the top of the display. iTunes Gift Card (US) is by far the most convenient way to credit to your US iTunes Store account.
Your updated balance appears in your US iTunes Store account once you redeem iTunes Gift Card (US). Each time you purchase content, iTunes deducts funds in the credit until it is depleted.
HOW TO REDEEM AN ITUNES GIFT CARD (US)?
1. Click the "Redeem" link in the "Quick Links" box of the iTunes Store
2. Type in the code, click "Redeem" and confirm "Redeem Prepaid Card"