Phosphodiesterase inhibitors are a class of drugs that block the action of phosphodiesterase enzymes, leading to increased levels of cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP) in cells, which can enhance signaling pathways and physiological responses. They are used in the treatment of various conditions, including erectile dysfunction, pulmonary hypertension, and certain inflammatory diseases, by modulating vascular tone, cardiac contractility, and inflammatory processes.