Cholesterol along with some other types of fat cannot be dissolved in the blood. In order for them to be transported to and from cells, they have to be specially carried by molecules called lipoproteins. These molecules consist of an outer layer of protein with an inner core of cholesterol and triglycerides (a form of fat). Lipoproteins are essential for cholesterol to move around the body. Mainly there two types of lipoproteins - High Density Lipoprotein (HDL) and Low Density Lipoproteins (LDL).
Low Density Lipoproteins carry fats to different parts of the body. LDL carry about 60-70 percent of the cholesterol around the body and thus called "bad" cholesterol. Studies show that excess cholesterol leads to much higher risk of heart attack and/or stroke. Although there are other factors involved in this risk such as age, gender, smoking, family history of heart disease, and diabetes mellitus, higher cholesterol is a major contributing factor that should not be ignored.
HDL, on the other hand, transports cholesterol from cells back to the liver. At this point, they are either reused or converted to bile acids and disposed. Therefore, HDL is called "good" cholesterol.
HDL actually helps fight the risk of heart attack and/or stroke. Because they consist of more protein than that of triglycerides or cholesterol, they work hard to remove LDL from your artery walls. When your physician checks your LDL cholesterol level, it is very possible that he or she will want to look at the HDL and triglycerides levels at the same time.