WHAT IS A DOOM/DOOM II DEMO (OR LMP FILE)?
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In the Doom/Doom II universe, the term "demo" refers to a
file that contains a recorded session of gameplay.
Demo files are primarily for use with PC-based Doom games.
Demo filenames have a ".LMP" file extension/suffix, and so
are sometimes referred to as "LMP files" or "LMPs".

Despite appearances to the contrary, a Doom/Doom II demo is
not a video file or "movie" of a game; It's simply a basic
data file containing encoded keyboard/mouse/joystick input,
which was recorded in precisely-measured intervals during a
live game.  This block of recorded input is stored between
a small game-information header and an end-of-file marker.

When a demo is "played back", an actual live game begins,
and control of the live player character(s) is provided by
the keyboard/mouse/joystick input recorded in the LMP file.

Random-number generation in the Doom/Doom II game engine is
directly influenced by signals from input devices.  When a
demo is played back, the recorded signals are read from the
file, and random-number generation is affected accordingly.
This is why monster behavior and other "random" events will
always occur the same way every time a given demo is played
back.

To record or replay demo files, a commercial version of the
compatible Doom/Doom II software is required.  (A demo file
is only an optional accessory to Doom/Doom II software, and
is not executable by itself.)

For best results when recording any demo, avoid pausing the
game, calling up menu screens, or entering cheat codes
while playing.  (Cheat codes and some other types of input
are not stored while recording demos, and this missing data
will often cause demos to go out of sync during playback.)

For best results when replaying a demo, be sure to use the
exact same version of Doom/Doom II that was originally used
to record the demo; otherwise the demo may fail to load, or
may go out of sync during replay.  For example, a Doom II
demo cannot be replayed using Doom 1; a demo recorded using
Doom II v1.9 cannot be replayed using Doom II v1.666.

For information on playing demos when using DOS or Doom 95,
click here.
For more about demo commands in DOS mode, click here.

If you are using a source port (a third-party Doom/Doom II
engine), note that demos made with the original engines may
go out of sync when they are replayed; Some source ports
will not load/replay/record original-format demos at all.
PrBoom is one of the more reliable modern source ports for
playing Doom/Doom II demos made with the original engines;
Chocolate Doom is another.

For notes on the demos available at this site, click here.