With stroke one, the piston is in the top position, the intake valve opens and the piston moves down to enable the engine to take in a cylinder-full of air and gasoline. Only the tiniest drop of gasoline needs to be mixed into the air for this to work.
With stroke two, the piston moves back up to compress the gasoline/air mixture.
With stroke three, the spark plug provides a spark to ignite the gasoline. The charge in the cylinder explodes, driving the piston back down again. This is also known as the power stroke, which is the main source of the internal combustion engine's torque and power.
With stroke four, the piston hits the bottom of its stroke, the exhaust valve opens and the exhaust gas is pushed out of the cylinder as the piston moves back up. The exhaust gas passes through the catalytic converter, exiting the vehicle through the tailpipe. With the completion of the cycle, the engine is ready to start the process all over again. In a car moving at 60 mph, it does this about 600 times per minute.