BTU (BRITISH THERMAL UNIT)
A British Thermal Unit (BTU) is the amount of heat energy needed to raise the temperature of one pound of water by one degree F. This is the standard measurement used to state the amount of energy that a fuel has as well as the amount of output of any heat generating device.
For example, take one gallon (8 pounds) of water and put it on your stove. If the water it 60 degrees F and you want to bring it to a boil (212 degrees F) then you will need about 1,200 BTUs to do this.
Propane - 15,000 BTUs per pound
Charcoal - 9,000 BTUs per pound
Wood (dry) - 7,000 BTUs per pound
This gives you an idea of how much fuel you'd need to cook something.
A home stove has on average 7000 BTUs per burner
Burners designed for low-heat cooking have about 3000-5000 BTU and others may go up to 12,000 BTUs.
A regular 6 burner professional range has about 12,000 to 20,000 BTU.
A wok range has about 90,000 to 150,000 BTU
All gas grills have a maximum BTU rating per hour. If you see a 35,000 BTU gas grill that means that that grill puts out 35,000 BTUs from all its main burners combined in one hour, or uses a little more than 2 pounds of propane an hour. The BTU rating on a gas grill doesn't necessarily tell you how much heat it will produce, but does give you a general idea of its heat output, and how much fuel you'll be burning.
BTU is mentioned in my page on Chinese Stir-Frying.