A four-stroke internal combustion engine has to do 4 things to complete one cycle.

The first cycle is the intake cycle. The crankshaft rotates and pulls the piston down in the cylinder which creates a partial vacuum in the cylinder. Since the intake valve is open, air is pulled through the carburetor where it also picks up fuel. At the end of the intake stroke, the camshaft rotates to a low spot on the lobe which allows the valve spring to close the intake valve.
The second stroke is the compression stroke. It compresses the fuel air mixture. During this stroke, both the intake and exhaust valves are closed.
The third stroke is the power stroke. Just before the piston/ crankshaft reach top dead center (TDC), the spark plug fires and the fuel/ air mixture is ignited. The heated gasses expand very rapidly and force the piston down (turning the crankshaft in the process). This the only part of the cycle where power is produced.
The last stroke of the cycle is the exhaust stroke. At the beginning of the stroke, the exhaust valve is opened by the camshaft. When the piston is forced back up by the crankshaft, the burned fuel/air mix is forced past the exhaust valve and into the exhaust port and pipe.