Dirt Bike FAQ

The four strokes refer to intake, compression, combustion and exhaust strokes that occur in a four stroke engine.
1 Intake: the fuel and air are brought into the cylindar
2 Compression: the fuel-air mixture is compressed.
3 Combustion:this mixture is ignited by the spark plug to repel the cylindar back and thus giving the energy to drive the engine.
4 The Exhaust stroke: The resulting fumes are expelled on the cylindars return.

In a two stroke engine the strokes combine these mechanisms so that on all four of these can be combined into two strokes.

You may want to check this out too: http://science.howstuffworks.com/two-stroke1.htm