.dm2 is the file format which is used to save
Quake 2 demos. By going to the
console in Quake 2 and typing
Record demoname
all of the information pertaining to whatever the player is in the vicinity of will be recorded into a .dm2 file which can be played back later using the command
map demoname
These files can also be edited by the use of a program such as
KeyGrip 2, allowing home computer game users to create their own
machinima movies.