Flames are algorithmically generated images and animations.
The software was originally written in 1992 and released as open
source, aka free software. Over the years it has been greatly expanded, and is now widely used to create art and
special effects. The shape and color of each image is specified by a
long string of numbers - a genetic code of sorts.
You can create your own flames with Fr0st (cross-platform and
scriptable in python), the Apophysis
interactive designer (for windows), Apophysis 7X the improved version, Qosmic (for
Linux), or Oxidizer (for Mac
OS X), or JWIldfire (java cross platform).
Or you can use the Electric Sheep distributed screen
saver to join the collective evolution and animation of flames.