As you might expect, there is no single file format for Internet radio. While the situation isnt as bad as it is for graphics (with literally hundreds of types), there are two major audio formats. The first is RealAudio from Real Networks, which is the de-facto standard for online audio. RealAudio is a well-developed format that is designed especially for streaming. It offers superior sound quality over various connection speeds.
The second major format is Windows Media, which is the default audio format on Windows. Windows Media isn't a bad format, but it's not as well-developed as RealAudio. And you really notice the difference in sound quality over different connection speeds. In this case, a faster connection really is better.
The other widely-used format is MP3. MP3 is a compressed audio format that offers near CD-quality sound, and which can be played back by most media players on any operating system.
Tuning In
Of course, all of this doesnt work without the proper software. This type of program is called a media player. To play RealAudio, you need the RealPlayer, which is a freeware application for Windows, the Macintosh, and Linux. To listen to Windows Media files, you don't need to download anything -- use the Media Player application that comes with Windows. You may need to upgrade to version 7.0 in order to listen to some content. You can do this automatically by selecting Check for Player Upgrades from the Help menu of the Media Player.
To listen to MP3 files, you can use the RealPlayer or Windows Media Player. Or, you can download a dedicated MP3 player like WinAmp or Apple's QuickTime player.