Cholesterol travels in the blood in packages called lipoproteins. Just like oil and water, cholesterol, which is fatty, and blood, which is watery, do not mix. In order to be able to travel in the bloodstream, the cholesterol made in the liver is combined with protein, making a lipoprotein. This lipoprotein then carries the cholesterol through the bloodstream.
There are specific kinds of lipoproteins that contain cholesterol in your blood, and each affects your heart disease risk in a different way.
Low density lipoproteins (LDL): the "bad" cholesterol. LDL carry most of the cholesterol in the blood, and the cholesterol from LDL is the main source of damaging buildup and blockage in the arteries. Thus, the more LDL-cholesterol you have in your blood, the greater your risk of heart disease. If you have heart disease or are at high risk for developing it and your LDL is 100 mg/dLor higher, your cholesterol may well be too high for you.
High density lipoproteins (HDL): the "good" cholesterol. HDL carry cholesterol in the blood from other parts of the body back to the liver, which leads to its removal from the body. So HDL helps keep cholesterol from building up in the walls of the arteries. If your level of HDL-cholesterol is below 40 mg/dL, you are at substantially higher risk for heart disease. The higher your HDL-cholesterol, the better. The average HDL-cholesterol for men is about 45 mg/dL, and for women it is about 55 mg/dL.
Triglycerides: a form of fat carried through the bloodstream. Most of your body's fat is in the form of triglycerides stored in fat tissue. Only a small portion of your triglycerides is found in the bloodstream. High blood triglyceride levels alone do not necessarily cause atherosclerosis. But some lipoproteins that are rich in triglycerides also contain cholesterol, which causes atherosclerosis in some people with high triglycerides and high triglycerides are often accompanied by other factors (such as low HDL or a tendency toward diabetes) that raise heart disease risk. So high triglycerides may be a sign of a lipoprotein problem that contributes to heart disease.
Source: the National Heart, Lung and Blood Institute (NHLBI) http://www.nhlbi.nih.gov/