Illustration of a lipoprotein, or cholesterol, molecule. It consists of a core of esterified cholesterol molecules and triglycerides surrounded by a shell of unesterified cholesterol and phospholipids. Lipoproteins can be either low density liproproteins (LDL), also known as 'bad' cholesterol, or high density liproteins (HDL), also known as 'good' cholesterol. When there are high levels of LDL cholesterol in the blood it builds up on the sides of blood vessels hardening them, a condition named atherosclerosis. This narrows the blood vessels and may block them. HDL cholesterol plays a role in fat metabolism and contributes to cardiovascular health.