Cholesterol is a necessary part of all membranes. However, many people have too much cholesterol circulating through their system. Because cholesterol is virtually insoluble in water or blood, circulating cholesterol is carried by a number of different transport proteins. These proteins are classed by their densities. The highest density protein (HDL) transports cholesterol to the liver, where it can be broken down and eliminated. Low density lipoprotein (LDL) transports cholesterol from the liver back to other tissues. HDL-bound cholesterol is considered "good" cholesterol because it's on its way out of the body. LDL-bound cholesterol is considered "bad" cholesterol because it's staying in circulation.
LDL−bound cholesterol is considered "bad" cholesterol because it's staying in circulation.