Let me give this a try.
A compression test uses a gauge that screws into the spark plug hole and test how many psi your cylinder makes on the compression stroke. Test the peak PSI. Your cyl may make a normal PSI but maybe leaking air buy after the cyl fires.
A leak down test uses compressed air(?) forced into the Cyl and sees how long it takes for the air to leak down to set PSI. This will let you know if the valves are seating right and if the rings are sealing good.
How did I do?
