Rather than download a big, chunky audio/video file all at once, streaming allows you to see and hear an audio/video file as it's transferred. Windows Media, RealNetworks, and QuickTime are currently the three most popular streaming media platforms. A player program (which are available for free) must be downloaded for each of these technologies in order to decompress audio/video files, which you can then listen to or view. Streaming video is usually sent from prerecorded video files, but they can be broadcast live as well. The FezGuys can tell you more about streaming media.