Displacement maps are grey scale textures.

Example displacement map for an MCM 1 Supercross track:

{{image url="http://dirttwister.com/eb/images/baatkips257.jpg" title="BaatKips SX 2000" alt="BaatKips SX 2000"}}

Example displacement map for a MCM 2 National track

{{image url="http://dirttwister.com/reflex/blender/valleyride_dispmap.jpg" title="Valley Ride MCM 2 Track" alt="Valley Ride MCM 2 Track"}}

Tutorials:

Basic Tutorials:

http://dirttwister.com/tutorials/overview.shtml

Advanced Tutorials:

http://www.twisteddirt.com/TutorialStuff/dem_01.php
http://www.twisteddirt.com/TutorialStuff/dem_02.php
http://www.twisteddirt.com/TutorialStuff/leveller_01.html