The stuff you buy at the iTunes Store is yours to keep(unless you rented it). You're not charged a monthly fee, and your digitally protected downloads don't go poof! after a certain amount of time. Nor do the songs come with such confusing usage rights that you need a lawyer to figure out if you can burn a song to a CD or not. Still, you must follow a few simple rules:
You can play downloaded songs on up to five different iTunes-equipped Macs or PCs (in any combination) and you can burn them onto CDs(seven times for each playlist). iTunes Plus tracks (page 159) are exempt from these rules.
You can watch movies, videos, and TV shows on any five computers, on as many iPods as you own, or piped over to the TV with an AV cable or via an Apple TV box.
A single iPod can host purchased items from up to five different accounts, but won't accept any files from a sixth account—a restriction designed to prevent a single iPod from filling up with copyrighted content purchased by, say, members of the entire sophomore class.
You can burn backup CDs and DVDs of your purchases, but you can't burn an iTunes movie or TV show to a disc and watch it on your DVD player. (On the flip side, some newer DVDs now come with an "iTunes Digital Copy" that you can add to your library from the disc; instructions are in the package.)