There are a variety of options for purchasing music online – iTunes, RealNetworks, Yahoo Music, Napster, and others. The files arrive encumbered by serious restrictions on your ability to play them, copy them, burn them onto CDs, and share them, but frequently the restrictions are not explained clearly.
The Electronic Frontier Foundation has assembled a User’s Guide to DRM in Online Music. It shows the marketing messages from the major services, then translates those words into plain English to make it clear just how crippled the music is when it arrives on your computer. It’s well worth reading before you consider spending money on online music.