To make the engine turn, a cycle of events must occur:
First an engine pulls in a mixture of fuel and air into the chamber above a piston. This process is called the "Intake Stroke"
Once the fuel/air is in the combustion chamber, a spark ignites the mixture. This is the "Combustion Stroke."
The liquid fuel heats up and turns into a gas pushing on all sides of the chamber which pushes the piston which is connected to a series of gears to deliver movement to whatever the engine is connected to! This is the "Power Stroke."
Finally, the engine dispenses waste gases via the "Exhaust Stroke."
These are the FOUR PROCESSES of the engine.
The up movement of the piston is one stroke. The downward movement is another stroke. A four-stroke engine completes all four of the processes listed above in two revolutions of the camshaft. That is the equivalent of four strokes!
A two-stroke engine completes all four engine processes in just one revolution of the camshaft (two-strokes)