Napster Mobile is an MP3 store on a phone. The only free aspect of the service is the ability to listen to 30-second previews of songs without having to buy them.
Songs bought via Napster Mobile are downloaded directly to your phone, assuming the phone has the storage space. This allows you to play music on your phone like you would an MP3 player.
Any song bought off of Napster Mobile can also be sent to your home computer via email, allowing you to have a copy not only on your mobile phone, but on your PC as well.
Not all cell phones are compatible with Napster Mobile. Phones that do work with the service include the Motorola V9, the HTC Touch and several models of the BlackBerry.
Napster Mobile is essentially a trimmed-down, free version of Napster's paid service Napster To Go, which allows you to stream and download unlimited music to your phone or other device without having to pay per track. Instead, you pay a monthly fee for unlimited access. If you stop paying the fee, however, the rights to listen to the music you downloaded will expire.