iTunes Gift Card is a full-fledged means of payment, with such cards you can pay for any purchases in the iTunes Store, App Store, iBookstore and Mac App Store without restrictions.

Using an iTunes Gift Card is similar to activating a promo code for an app. On an iPad, iPhone, or iPod Touch, launch the App Store and at the bottom of the Featured tab, click Enter Code and rewrite the card code.

You can redeem a gift card from your computer in iTunes. In the iTunes Store tab at the very bottom of the page, select "Enter Code". And activate the card code.

Official gift card activation instructions:
http://support.apple.com/kb/HT1574?viewlocale=ru_US