Digital music players can store 10,000 songs or more. You could go broke filling it up by paying a dollar per song online. But that's not the only issue. Choose where you purchase music wisely or the songs may not play.

There are two basic types of legal online music services. Music stores allow you to purchase and download songs. Or you can rent songs from a subscription service.

It sounds simple, but it's not. Most online stores encrypt songs to stop illegal file sharing and duplication. The most-widely used formats are Advanced Audio Coding (AAC) and Windows Media Audio (WMA). The popular Apple iPod plays AAC files. Most other players use WMA.

Apple iTunes

The Apple iTunes music store has over 2 million songs in its library. Each song costs 99 cents; albums are $9.99 and up. The songs are encoded in the AAC format. The iTunes music store runs on both Windows and Macs. It integrates seamlessly with the ubiquitous iPod and its various iterations.

But music purchased from iTunes does not easily transfer to other brands of music players. If you don't own an iPod, you could wind up with a lot of music on your computer's hard drive that isn't playable on your portable machine.