Further we'll only be concerning ourselves with poppet type valves. Seat Is the ground area of the cylinder head which matches the shape of the face. Together the Face and the Seat provide the tight gas tight seal necessary for engine operation. Usually a cam turns and lifts the stem which in turn lifts the valve.
Guide Is a tube in which the stem moves.
Together the guide and stem are such close tolerance that they provide an oil and air tight seal. Spring Holds the valve closed against the vacuum present in the cylinder during various parts of the stroke. For example, you do not want the intake valve to open during the power stroke, nor do you want the exhaust valve to open during the charging stroke. When an engine is new the fits are carefully machined and in close tolerance to each other. Over time the engine wears and this tolerance is lost.