Cholesterol is a fatty substance made by your liver. And despite its bad reputation, you actually do need it. It helps to form cell membranes and some of your hormones.

Your body makes all the cholesterol it needs. But when you eat foods high in saturated fat, it triggers your liver to churn out more cholesterol, much more than you require. That excess cholesterol builds up in your arteries, blocks bloodflow and, if left untreated, can set the stage for a heart attack or stroke. When we say you have high cholesterol, it means that the total amount of cholesterol in your bloodstream is above a certain number that experts have deemed a risk to your health.

But there’s more than just your total cholesterol. There are actually two types of cholesterol—low-density lipoproteins (referred to as LDL) and high-density lipoproteins (called HDL). Lipoproteins are combinations of fat and protein made by your body. They are responsible for delivering other fats, which don’t dissolve in your blood, to your cells.

What doctors have realized in recent years is that your LDL and HDL levels may be just as—if not more—important as your total cholesterol reading. Each type has a different action, and therefore a different impact on your heart health.

LDL, often called the “bad” cholesterol, carries most of the cholesterol in the blood. When there’s too much LDL, it can combine with other substances to form a plaque—a hard, thick deposit that sticks to artery walls. If a blood clot develops in the vicinity of a plaque, the reduced bloodflow can lead to a heart attack or a stroke. Experts now think that the lower your LDL level, the better.

HDL is known as the “good” cholesterol. It carries about one-fourth to one-third of the cholesterol in the blood, transporting it from different organs to the liver for disposal from the body. HDL may also pick up cholesterol deposited in your arteries and move it on out, preventing it from building up in your arteries.