Cholesterol is not very soluble in water. In order to move around our bloodstream it must be carried in some form of transport molecules. This comes in the form of certain proteins called apoliproteins that wrap around cholesterol and other blood fats to form lipoproteins - essentially a combination of protein and fat.
The largest liproprotein is called Chylomicrons. They mainly transport fat from the intestines to the liver. Then there is Very Low Density Lipoproteins (VLDL) - mad in the liver - that transport triglycerides to various tissues such as muscles for for energy production and body fat for storage.
The most famous of cholesterol is Low Density Lipoproteins (LDL) also known as "bad cholesterol". When too much LDL (bad) cholesterol circulates in the blood, it can slowly build up in the inner walls of the arteries that feed the heart and brain. Together with other substances, it can form plaque, a thick, hard deposit that can narrow the arteries and make them less flexible. This condition is known as atherosclerosis. If a clot forms and blocks a narrowed artery, heart attack or stroke can result.It is the major carrier of cholesterol and triglycerides taking them from the liver to other parts of the body.
Lesser known is Tryglycerides and Lipoprotein (a). Triglycerides is a storage form of fat. Both excess carbohydrates and fats are converted into tryglycerides in the liver. Elevated triglycerides can be due to overweight/obesity, physical inactivity, cigarette smoking, excess alcohol consumption and a diet very high in carbohydrates (60 percent of total calories or more). It is a major risk factor for heart disease.
Lipoprotein (a) is like LDL cholesterol except that it carries lipoprotein (a) a sticky repair protein that can thicken the walls of the arteries. Lp(a) isn’t fully understood but a high level of Lp(a) is a significant risk factor for the premature development of fatty deposits in arteries. Like LDL and Triglecerides it is a major risk factor for heart disease.
High Density Lipoprotein (HDL) on the other hand takes cholesterol from various parts of the body to the liver to be excreted in bile. About one-fourth to one-third of blood cholesterol is carried by high-density lipoprotein (HDL). It is high in protein and low in cholesterol which makes it ideally suitable for the job. A high levels of HDL seem to protect against heart attack while low levels of HDL (less than 40 mg/dL for men, less than 50 mg/dL for women) also increase the risk of heart disease. There is evidence to suggest that HDL cholesterol cleans out excess cholesterol from the walls of the blood vessels.
In addition HDL also carries antioxidant enzymes and vitamins and prevent the oxidation of LDL cholesterol. What is oxidized LDL? Free radical can damage LDL cholesterol making them into oxidized LDL. These oxidized form of LDL are more likely to promote damage to the inner lining of the arteries and atherosclerosis.
This is why HDL cholesterol is known as good cholesterol.