Music that is offered online legally comes in a particular format. An audio file format includes both the way the audio is stored and the way that any digital rights management system is added. These determine which computer systems, phones, or a portable music players can play the file.

There are several audio formats that support music files, MP3 being the dominant one. MP3 files are supported by almost all music players but are rather larger than the other formats. AAC is best known for playing on the iPod but is supported on other players. WMA (Windows Media Audio) plays on a wide range of other portable players and smartphones.

Some files are protected by digital rights management - Fairplay for the iPod, Windows Media DRM for WMA players. Because these do not work with each other, some stores have started to offer non-DRM tracks alongside DRM ones, allowing consumers to buy music from any store, take their tracks wherever they want and play them on pretty much any device.

The digital music business is more than pay-per-download though, and DRM is still important in order for certain other business models to exist, particularly subscriptions (where users pay a monthly fee for unlimited access to music), super-distribution (allowing fans to share music with their friends) and time-limited downloads (such as those offered by ad-supported services).

Click here for a detailed list of formats (wikipedia)