This is a pretty good explanation....




Since cholesterol is a fat, it can't travel alone in the bloodstream. It would end up as useless globs (imagine bacon fat floating in a pot of water). To get around this problem, the body packages cholesterol and other lipids into minuscule protein-covered particles that mix easily with blood. These tiny particles, called lipoproteins (lipid plus protein), move cholesterol and other fats throughout the body.

Cholesterol and other lipids circulate in the bloodstream in several different forms. Of these, the one that gets the most attention is low-density lipoprotein— better known as LDL, or "bad" cholesterol. But lipoproteins come in a range of shapes and sizes, and each type has its own tasks. They also morph from one form into another. These are the five main types:

Chylomicrons are very large particles that mainly carry triglycerides (fatty acids from your food). They are made in the digestive system and so are influenced by what you eat.
Very-low-density lipoprotein (VLDL) particles also carry triglycerides to tissues. But they are made by the liver. As the body's cells extract fatty acids from VLDLs, the particles turn into intermediate density lipoproteins, and, with further extraction, into LDL particles.
Intermediate-density lipoprotein (IDL) particles form as VLDLs give up their fatty acids. Some are removed rapidly by the liver, and some are changed into low-density lipoproteins.
Low-density lipoprotein (LDL) particles are even richer in pure cholesterol, since most of the triglycerides they carried are gone. LDL is known as "bad" cholesterol because it delivers cholesterol to tissues and is strongly associated with the buildup of artery-clogging plaque.
High-density lipoprotein (HDL) particles are called "good" cholesterol because some of them remove cholesterol from circulation and from artery walls and return it to the liver for excretion.
– By Julie Corliss
Executive Editor, Harvard Heart Letter