it has four fused ring systems (three six membered rings and one five membered ring) called the A, B, C, and D rings.  The B ring has a double bond, and A ring has a hydroxyl group (alcohol), and the D ring contains an aliphatic hydrocarbon chain.

Since cholesterol is very non-polar, it solubility in water in minimal.  This means that its solubility in blood is also minimal.  As a result, cholesterol is transported through the bloodstream by lipoproteins, polymers that contain lipids (fats) and proteins.  The density of these lipoproteins determines the amount of cholesterol that they can carry.  LDL (low density lipoprotein) carries a higher percentage of cholesterol while HDL (high density lipoprotein) carries a smaller percentage.  So HDL in considered the good cholesterol and LDL the bad cholesterol.  But the actual cholesterol itself in the lipoproteins is the same no matter the density level.  Chemically there is no difference between the two.

Cholesterol is an important body molecule; it is a key component of cell membranes and a precursor to a number of hormones and to vitamin D. Cholesterol is synthesized in the human body by the liver, and may also be consumed as part of the diet; on average, about 75%-85% of one's total body cholesterol is made within the body, and the remainder is obtained from food.

The cholesterol molecule is made up of a series of carbon rings; there are three 6-carbon rings side by side, with a 5-carbon ring on one end. The molecule has the formula C27H46O.

The body transports cholesterol by producing special proteins which attach to the cholesterol molecules; the combination of protein and cholesterol is known as a lipoprotein. HDL, or High Density Lipoproteins, is sometimes called "good" cholesterol, because HDL travels to the liver, where the cholesterol is converted into bile and excreted from the body. This results in lower levels of cholesterol in the bloodstream.

LDL, or Low Density Lipoproteins, work to keep cholesterol circulating in the bloodstream. This can cause problems when the endothelium, or lining of a blood vessel, is damaged. Endothelium damage (which can be caused by a variety of things, including smoking, hypertension, diabetes, or bacterial infection) creates thin spots where LDL can leave the bloodstream and lodge in the wall of the blood vessel. White blood cells may move into the area to try to digest the cholesterol, resulting in even more damage to the endothelium, and over time a plaque can build up. Plaques narrow the arteries and impede blood flow, putting the patient at risk of a heart attack or ischemic stroke.

Cholesterol in the diet comes exclusively from animal sources. Meat, dairy, and egg yolks are the main culprits for dietary cholesterol. People on a low cholesterol diet need to avoid fatty cuts of meat, liver, butter, full-fat milk products, and egg yolks. Lean meats in moderation, skim milk and products made from skim milk (yogurt, etc), and egg whites are acceptable.

The sterol in question here is cholesterol, which is needed in the body for lots of different functions, from cell membranes to joints in the skeletal system.  Cholesterol comes in two forms, high density cholesterol (HDL) and low density cholesterol (LDL).  Of the two, HDL is the better of the two, as the LDL contributes directly to the formation of atheromas inside the arteries.  HDL serves to dissolve the atheromas in the arteries, so it is better to have a higher HDL level and a lower LDL level.  For this reason, HDL is sometimes referred to as the "good cholesterol". while the LDL is called the "bad cholesterol".  Food sources for cholesterol include animal fats, oils, dairy products and various plant sources.  Diets that restrict the intake of fats and oils are advisable for people who are at risk to develop coronary artery disease.  Coronary artery disease (CAD) is the leading killer among people older than 20 in the western hemisphere.

The sterol which is contributing to his high risk for coronary artery disease is cholesterol.  There are said to be two major forms of cholesterol.  LDL is the bad form and HDL is the good form.  The reality is that the cholesterol itself is the same in both forms.  The difference between the two is the density of the cholesterol contained in the lipoproteins that transport them through the bloodstream.  LDL's have a higher percentage of cholesterol than HDL's.  Another difference between the two is that HDL's help to bring cholesterol to the liver for chemical processing, thereby reducing the cholesterol content in the blood.

Cholesterol in general is found in any food containing animal fats.  Meat and dairy (eggs and cheese in particular) contain cholesterol.  Plant matter (fruits and vegetables) contain very little to no cholesterol.  One particular bad food source linked to elevated LDL's are trans fats.  Trans fats are mostly created in chemical processes and are therefore mostly found in highly processed foods like fast food and snack foods.