A two stroke internal combustion engine, like the name implies, has only two strokes - compression and expansion (power). Since power is made once per revolution (as opposed to once per two revolutions with a four-stroke engine), a two-stroke has the potential to make twice as much power per unit displacement or weight as a four-stroke. Inefficiencies inherent to the two-stroke system reduce the power boost to 1.8 times more, but it is still substantial.
Instead of valves, two-stroke engines use ports on the side of the cylinder to let in air and expel exhaust. Since there is no intake stroke, the incoming charge must be either compressed by the crankcase or a supercharger. If the crankcase is used, then there can be no dedicated oil system, and oil must therefore be added to the fuel. Two-stroke engines are commonly found in mopeds and chainsaws, because weight is important and mopeds usually have displacement limits. Two-stroke engines are very polluting; some of the fuel goes unburned, and the burning of the added oil causes blue smoke.