BTU stands for British Thermal Unit - a measure of heat energy. It is 'British' because it uses 'Imperial Gallons' in its calculation. An Imperial Gallon of water weighs 10 pounds. A BTU heats one pound of water to one degree Fahrenheit.
This is an old measurement that has been replaced in the metric system, but it still remains the convention used in the field of HVAC (heating, ventilation, air conditioning). One BTU can also be deducted from a pound of water to lower its temperature by 1ºF. Although air conditioning involves changing the temperature of the air and not water, the principle is the same.
The BTU figure is the one you need to compare air-con systems. Domestic systems usually range from 9000btu to 24000btu, and the size of room dictates the power of system it requires.
The Penguin website has a calculator to asses the BTUs you need for your room