Cholesterol is a a compound found in most body tissues. Too much of it is associated with heart disease and heart failure, but the substance itself can promote health if managed properly.
There are two man kinds of cholesterol - Low-density lipoproteins (LDL) are sometimes called “bad” cholesterol. High levels of LDL cholesterol can build up in your arteries, causing heart disease. High-density lipoproteins (HDL) are referred to as “good” cholesterol.
Cholesterol is a type of lipid, just as fats are. However, unlike fat, cholesterol can't be exercised off, sweated out, or burned for energy. HDLs and LDLs are found only in your blood, not in food. Genetic factors are the most common cause for high levels of LDL (“bad” cholesterol) and triglycerides.
"Molecule for molecule, cholesterol can make up nearly half of the cell membrane. Since it is smaller and weighs less than other molecules in the cell membrane, it makes up a lesser proportion of the cell membrane's mass, usually roughly 20 percent."
All types of lipoproteins contain both lipids and proteins, but the relative composition of each lipoprotein varies. The main structural difference between LDL and HDL is their compositions. Approximately 50 percent of the weight of an LDL particle is cholesterol and only 25 percent is protein. High-density lipoprotein particles, on the other hand, consist of 20 percent cholesterol by weight and 50 percent protein. - http://healthyeating.sfgate.com/
During a blood test, LDL and HDL are both monitored along with other things because they are used to help evaluate the patient's risk of heart disease. Whether more cholesterol is being taken to or from cells can be determined during a blood test. HDL levels should be at least 40mg/dL or closer to 60mg/dL. LDL should be lower than 129mg/dL or even lower for people that are at higher risks for heart disease.
An excess of LDL can result in plaque buildup on arterial walls which can ultimately result in atheroscerlosis.
A healthy diet with reduced fat and cholesterol will increase HDL levels and decrease LDL levels. Saturated fat intake should be limited 7% or less of total calories, cholesterol should be 200mg per day or less, and Omega 3 fatty acids will increase HDL levels.
Doctors will also measure triglycerides, blood sugar level, and total cholesterol.