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THE STUFF YOU BUY at the iTunes Store is yours to keep (unless you rented it). Apple doesn’t charge you a monthly fee, and your digitally protected downloads don’t go poof! after a certain amount of time.
You do have to cope with restrictions on some iTunes purchases, namely anything that’s copy-protected. Copy-protected media include movies, TV shows, music videos, and songs you bought before April 2009, when Apple still copy-protected its music files (it no longer does; see below). For those types of files, you’re bound by the iTunes Store usage agreement: