What is the difference between mitral regurgitation and mitral stenosis?

Blockage vs leaking: In stenosis the valve opening is to small which interferes with forward flow. Regurgitation is a leaking of the blood backwards. Not uncommonly both occur in the same patient.

Loose vs Tight: The mitral valve directs blood flow from the left atrium to the left ventricle. When the lv squeezes the mv prevents blood from leaking back into la. If the mv fails to completely close and blood leaks back this is mitral regurgitation or insufficiency. When the mv is stiff and doesn't open enough and slows down the flow into the lv this is mitral stenosis. A mv can also have both at the same time.