BTU (British Thermal Unit) is a measurement of energy. One BTU is the amount of thermal energy required to raise the temperature of a pound (0.45kg) of water by one degree Fahrenheit (0.56 degrees Celcius).
Most manufacturers list their stove output in BTUs per hour. The higher the BTUs, the hotter the stove. For comparison a candle flame puts out about 100-300 BTU/hr, a backyard barbeque creates 25,000 BTU/hr. Stoves generally operate somewhere between 3000 and 10,000 BTU/hr. But, these measurements reflect stoves operating at maximum output and in perfect conditions. In the field, cold, wind, altitude, and carbon deposits in your stove can reduce its output. Pressure within the fuel bottle or canister will also effect the amount of heat generated.
Stove descriptions also include the time required to boil a litre of water. Hotter stoves can generally produce faster boiling times, but there other factors that contribute to field performance. How well the stove reflects heat, and whether the stove can be effectively screened from wind will effect boil rates. Canister stoves that attach directly to the burner cannot be enclosed with a wind screen as reflected heat can dangerously overheat the pressurized canister. Alcohol stoves require air intake, they can actually be less efficient when tightly screened.
When comparing figures, keep in mind, the stove with the fastest boil time may not be the most fuel efficient. And, if you're more likely to be simmering a pot of sauce, rather than melting large quantities of snow, having the hottest flame might not be your first concern.
As a very general guideline, liquid stoves burn approximately 120ml (4oz.) of fuel per person per day for quick-cooking meals and hot drinks. If you're melting snow as well as cooking, you should double this estimate. Extreme cold and high altitude will increase the fuel required.
One medium sized LPG canister (225-250g) will generally provide fuel for two people over three days in summer conditions. It's best to pack extra fuel until you're sure of your own requirements.
Read Camping Stove Fuel for information about the benefits and drawbacks of each stove design.