Cardiology is sub-specialty of Internal Medicine. Cardiologist or Heart Specialist treat and prevent various diseases and conditions related to heart, blood vessels as well as the circulatory system, including diagnosis and treatment of congenital heart defects, coronary artery disease, heart failure, valvular heart disease etc.
Interventional cardiologists additionally specialize in procedures like angiography, angioplasty etc. Surgeons who specialize in cardiac surgery are called cardiothoracic surgeons or cardiac surgeons, which is a sub-specialty of general surgery.