A DEM file is a game replay file created by various first-person shooter games and most popularly by Counter-Strike (supported by Valve's Source game engine). DEM files are used to record a player's perspective in a single or multiplayer game, which can then be shared with others.
To play a Counter-Strike DEM file in Windows, first place it in the cstrike\ installation directory. Next, start Counter-Strike and open the console window. From the console window, type viewdemo demoname, where "demoname" is the filename without the ".dem" extension.
DEM files are also used by Hexen II, Quake, and Unreal Tournament. However, the files are only playable on the game for which the demo was created.
NOTE: DEM files created by Source engine games, such as Counter-Strike, can be opened in VirtualDub to play the video frame sequence and record it to other output formats, such as .AVI. There is a size limitation of 2 GB for AVI videos created by Valve's source engine. You can install the Apple QuickTime component to encode the .H264 codec in order to avoid the size limitation.