there are two different types of internal combustion engines. a two stroke and a four stroke.
in a four stroke engine, such as a full size car engine - there are four things that happen.
1. intake
2. compression
3. combustion
4. exhaust
each of these things happen individually during each stroke of the piston. on the down stroke(1) it sucks in fuel and air, on the up stroke(2) it compresses the mixture, on the down stroke(3) the mixture explodes, or combusts, and on the up stroke(4) it pushes the spent gases out the exhaust valve outlet. thus it takes 4 STROKES of the piston to complete one cycle. This is where the name four stroke, or four cycle engine comes from.
In a two stroke, as the piston goes down, it sucks in the fuel/air mixture and and fires the mixture at the same time, and then on the up stroke it expells the gases and compresses the new mixture at the same time. thus it only takes 2 strokes, or cycles of the piston to create one complete cycle.
a two stroke is usually more powerful, but a 4 stroke is much more fuel efficient.
btw- for the sake of argument, there is a third internal combustion engine, known as a rotary, that uses triangle shaped rotors instead of pistons, but this system is rarely used today.
hope this helps
