Cardiology is a specialty of internal medicine (the study
of organs inside the body and diseases of these organs)
that involves studying the structures, functions, and
disorders of the heart and blood vessels, as well as the
diagnosis and treatment of such disorders. Blood vessels
are tubes in the body that carry blood. Since heart
disorders lead to the death of many people every year,
cardiology is a large part of the training for virtually all
physicians (medical doctors).
Although many family practitioners are familiar with common disorders of the heart,
medical doctors who specialize in cardiology known as cardiologists. To become a
cardiologist in the United States requires a physician to study three years of internal
medicine and three years of cardiology under the supervision of a another physician.
This period of study is known as a medical residency. 

FEATURED BOOK: Clinical Cardiolgy Made Ridiculously Simple (2014)

There are four main subspecialties of cardiology. The first is echocardiography, which is
a type of ultrasound study that provides a clear picture of the heart and also measures
blood flow in the heart. Cardiac electrophysiology is the study of the electrical
characteristics of the heart and disease that effect electrical conduction in the heart.
Interventional cardiology is the use of catheters for the treatment of structural diseases
of the heart and diseases caused by decreased oxygen to the heart.
"Where Medical Information is Easy to Understand"™
A catheter is a flexible, hollow tube that is inserted
into an opening of the body, blood vessel, or duct
in the body, with the main purpose of allowing
fluids, gas, or surgical instruments to pass from or
into these areas. Finally, there is nuclear
cardiology in which a radioactive substance is
injected into the body to visualize the heart to help
diagnose and treat heart disease. Cardiology
comes from the Greek word "kardia" meaning
"heart," and the Greek word "logos" meaning "the
study of." Put the two words together and you
have "study of the heart."