Ernie and Erica Wisner of ernieanderica.info say:
"Rocket stoves are a type of fuel-efficient device, named in the 70's, but dating back millenia in concept. A super-hot chimney right above the fire draws not only smoke but flame up quickly, blending hot fuel and air into a quick, hot, clean-burning fire that takes little wood, leaves little residue, and has lots of uses.
The Rocket Mass Heater takes this useful, clean-burning heat, adds a self-feeding wood box, and channels the exhaust through touchable thermal mass (as seen in Roman hypocausts, Chinese 'kang' stoves, northern European masonry heaters, and ancient masonry dwellings and hearths)."
Ianto Evans, Leslie Jackson, Ernie Wisner, Kirk Mobert, Paul Wheaton, and other friends are among the researchers developing efficient, clean-burning, affordable stoves for a variety of heating and cooking needs. This group developed and published the current editions of Rocket Mass Heater, or Rocket Masonry Stove, designs. The technology combines an innovative clean-burning combustion chamber, with an earthen masonry thermal mass, resulting in extraordinary heat capture and low waste from an incredibly small amount of wood.
Paul Wheaton, starts his extensive article on the RMH by saying:
rocket mass heaters in a nutshell:
heat your home with 80% to 90% less wood
exhaust is nearly pure steam and CO2 (a little smoke at the beginning)
the heat from one fire can last for days
you can build one in a day and half
folks have built them spending less than $20
And going with the whole 'picture is better than words' thingy here are a couple of explanatory pics stolen from the links above. In these examples the brown butt-warming area is made of cob, which is a mixture of clay, sand and straw
All links posted so far have a ton of information including text, diagrams, photographs and video. Ernie and Erica have even started selling detailed plans via their site. If that isn't enough, there is also this permies.com subforum largely dedicated to the RMH and there is always good ole google if you're still hungry for more.
One way or the other I've got to have me a rocket mass heater.