Kobus Venter put together a great article that helps define the Rocket Stove. Please take a look at it on his web site: http://vuthisa.com/2011/03/21/what-is-a-rocket-stove/
To paraphrase a couple of key points:
Dr. Larry Winiarski, now Technical Director of Aprovecho, began developing the Rocket stove in 1980 and invented the principles of the Rocket stove in 1982. The Winiarski Rocket stove’s simple design and use of common materials make it easily modified for optimal performance.
A Rocket stove’s main components are:
Fuel magazine: Into which the unburned fuel is placed and from where it feeds into the combustion chamber. The fuel magazine can be horizontal where additional fuel will be added manually or vertically for automatic feeding (gravity feed) of fuel. The fuel magazine can be simple steel piping or even ceramic pipe. Fuel shelves serve as the platform for the fuel that is used with the stove. This slightly raised platform makes it possible for air to flow over and under the fuel source.
Combustion chamber/Internal chimney: At the end of the fuel magazine where the wood is burned. Internal chimneys are mere extensions of the combustion chamber and may be constructed from a larger tin can to piping and provide the required draft to maintain the fire. The top of the combustion chamber/chimney serve as the support for the cooking area. Some Rocket stove designs have chimneys in a separate location to the combustion chamber.
Chimneys: Located above the combustion chamber or to one side or can be part of the hood extraction system.
Heat exchanger: To transfer the heat to where it is needed, i.e. the cooking pot. From the chimney the heat passes into a suitable heat exchanger to ensure the efficient use of the generated heat. For cooking purposes the design keeps the cooking vessel in contact with the fire over the largest possible surface area by use of a pot skirt to create a narrow channel which forces hot air and gas to flow along the bottom and sides of the cooking vessel. The pot is usually encompassed by a fixed or adjustable pot skirt. The pot skirt functions as a shield to force the emission gases to pass close to the container holding the food. The gap between the skirt and the pot is also known as the pot gap. The pot gap calculation is crucial to the performance of the stove and excel spreadsheets are usually used to calculate this gap.