Use the adjective cardiovascular when you're talking about the circulatory system in general or the heart specifically. Cardiovascular exercise gets your blood pumping and is good for your heart.
Cardio- means "heart," from the Greek kardia, and vascular refers to blood circulation, from a Latin root meaning "vessels or tubes." So anything cardiovascular is related to the heart muscle and the blood it pumps through arteries, veins, and capillaries. Cardiovascular diseases affect the structure of the heart itself or the vessels that help it move blood through the body. Things like not smoking and regularly moving your body can help you keep your cardiovascular system healthy.