I think he was looking for what they are not how they are used...

A demo file: Is a recording of a video game using that games integrated recording system. This is not to be confused with a video as a demo is more like a saved game file. When you watch a demo you must have the map that the demo was played on and you actually load all the data from the demo.

As a result to record a demo you need any modern game.

To watch a demo you simply need that same game and the correct map.