Heart failure is a clinical syndrome where the heart cannot maintain adequate cardiac output to meet the body's needs. It has multiple causes, commonly due to ischemic heart disease or hypertension, and is a progressive disorder. Heart failure is classified as acute or chronic, with acute having a rapid onset and requiring urgent treatment, while chronic is a progressive cardiac dysfunction from structural abnormalities. It is also classified as systolic or diastolic, with systolic having a reduced ejection fraction and diastolic having impaired ventricular relaxation or filling.