All online music stores and services are not created equal. They work with different MP3 players, and they sell and stream music with different file types and bit rates. It's this last quandary that we're covering here. First, note that file types and DRM (in other words, copy-protected files) are infinitely important when it comes to devices such as MP3 players. For example, only the iPod currently supports protected AAC files, whereas a wide selection of MP3 players supports protected WMA files. All MP3 players play MP3s, of course. If you listen to music on only the computer, you should have no problems finding a software jukebox that will play any of the file types below; in fact, most online music services require that you download their jukeboxes to purchase and play music. When it comes to sound quality, WMA, AAC, RAX, and ATRAC3 files are roughly equal when encoded at the same bit rate. MP3 sound quality is roughly half as good as the aforementioned file types, so an MP3 file encoded at 320Kbps would be about equal with a WMA file encoded at 160Kbps. Keep in mind that the higher the bit rate, the better the sound quality and the larger the size of the file (within the same file type). The chart below should help you navigate the jungle that is legal online music.

Note: The streaming bit rate refers to the files that are continuously streamed over the Internet as part of a service for which you pay a monthly subscription, not 30-second sample streams.