Hello kawawaka,
Thanks for using Apple Support Communities.
For more information on this, take a look at:
iTunes Store, Mac App Store, and iBooks Store: About your Apple ID and password
http://support.apple.com/kb/ht2204
You can use your Apple ID and password to sign in, buy content, and authorize items you've purchased in the iTunes Store, Mac App Store, or iBooks Store.
Best of luck,
Mario