The Piston is going down so the volume of the cylinder is getting larger. The Intake valve is open. A vacuum, or pressure of less than atmospheric is created in the cylinder.
Atmospheric pressure forces air into the cylinder. 14.7 psi at sea level. Fuel is mixed with the air, and the cylinder is filled with the mixture of air and fuel.
Compression Stroke
The Piston is going up so the volume of the cylinder is getting smaller.Both valves are closed. Air/fuel mixture is trapped in the cylinder. Pressure is increasing. Mixture is compressed into approximately 1/8 it's original volume. 150 psi built up. As the mixture is compressed, it heats up.
Power Stroke
The Piston is going down. Volume of the cylinder is increasing. Both valves closed. Spark plug fires, causing compressed air/fuel mixture to burn. The flame front travels across the combustion chamber. The burning fuel creates tremendous pressure which forces the piston down. turning the crankshaft and producing power. This is not an explosion, but a smooth, controlled burn.
Exhaust Stroke
The Piston is going up, so the volume of the cylinder is getting smaller. The Piston pushes the burned exhaust gasses out past the open exhaust valve, and out through the exhaust system and into the atmosphere. Both valves are open for a short period of time at TDC. This is called "Valve Overlap" The intake stroke of the new cycle follows this exhaust stroke.
Engine terms
Top Dead Center (TDC) - Point of uppermost travel of the piston and crankshaft.
Bottom Dead Center (BDC) - Point of lowermost travel of the piston and crankshaft.
Bore - Diameter of the cylinder.
Stroke - Distance the piston travels on a stroke, or the distance from TDC to BDC. The stroke is twice the distance of the throw.
Throw - The distance from the center of the main journal of the crankshaft to its rod journal. The throw is one half the distance of the stroke.