Compression ratio is the total volume of the combustion chamber and cylinder when the piston is at the bottom of the stroke and when the piston is at the top of it’s stroke.
Higher compression ratio usually produces more power but does require higher octane fuel.
Engine displacement is how the internal size of the engine is described. This is measured by the cylinder bore and by the length of the stroke and adding all the cylinders.
Some engines have one intake and one exhaust valves while some engines have two intake and two exhaust valves.
The valves are controlled by the camshaft. Some engines have two camshafts.
Overhead Valve OHV
Is when the camshaft has rocker arms arms or push rods actuating all of the valves.
Single Overhead Camshaft SOHC
A SOHC design means the camshaft is over the exhaust and intake valves.
Double Overhead Camshaft DOHC
In a double overhead camshaft design, there is a camshaft for the intake and a camshaft for the exhaust and both are above the valves.