A viewer is a program which can load and display a document of a particular type, such as a graphic, "movie" or a sound clip. Web browsers use external viewers to display media which the Web browser itself has not been programmed to display. You can configure your Web browser to launch any viewer based upon the file extension of the fetched file. The viewer doesn't have to know anything about the World Wide Web or Mosaic; it just has to be capable of taking the file to be displayed as a parameter at the time it is invoked.

For an example, imagine a new Windows application called "vreal" comes on the market which displays virtual reality spaces or scenes, allowing the user to navigate around the scene looking at whatever they desire, and these spaces are stored in files with a .vr extension. You could configure your Web browser such that any time it was told to fetch a url which pointed to a file with a .vr extension, it would use vreal to display it. When you clicked on a hotlink which referenced a file with a .vr extension, your browser would fetch the file, for instance scene.vr, store it in a temporary local file, then launch the viewer with the local file as a parameter: vreal scene.vr. In this way, Web browsers are extensible to new media with out any requirement of a change to the browser software itself.