The explosion cycle that takes place in the engine has four stages or strokes and the pistons go through these four stages repetitively, at very high speed. The four stroke cycle takes place within each individual cylinder.

1: Intake

As the piston moves down, the intake valve opens and lets in the fuel / air mixture.

2: Compression

The piston moves back up and compresses the gas mixture.

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3: Power

The compressed gas is ignited by the spark from the spark plug. The gas explodes and pushes the piston back down generating the power which is harnessed to drive the car.

4: Exhaust

The piston returns upwards, opening the exhaust valve and letting the exhausted burned gas out into the exhaust system.

The power is harnessed in stroke #3. As the explosion takes place the piston pushes one of the major components of the car. This is the crankshaft. This is the shaft that connects and transfers power from the engine to the transmission.

The piston attaches to the crankshaft by means of a connecting rod. The piston, rod, and crankshaft are joined together in a way that causes the crankshaft to revolve at high speed.
The entire purpose of all the prior actions that have taken place in the car, the battery, the starter, the spark from the spark plugs, the fuel and air from the carburetor, the four-strokes of the engine is to make this crankshaft move.

The four-stroke cycle takes place within each combustion chamber. Your car has several cylinders (usually four or more). The different cylinders are lined up to perform the four stroke cycle one after the other in a specific, precisely timed sequence. This sequence can vary slightly depending on the number of cylinders and the make of car. The four-stroke cycle within any one cylinder always occurs in the sequence of strokes outlined above.