Two fundamental defects:
Valve incompetence/insufficiency - Valvular insufficiency results from the valve leaflets not completely sealing when the valve is closed so that regurgitation of blood occurs (backward flow of blood) into the proximal chamber.
Valve Stenosis - Failure of a valve to open fully impeding forward flow.
These defects can lead to an array of further issues such as hypertension (high blood pressure), oedema (collection of intracellular fluid), hypertrophy of cardiac walls (causing reduced mechanical functionality) etc.