Cholesterol travels in the blood transported in molecules called lipoproteins. These are sphere shaped assemblies (containing lipids and proteins) that keep the cholesterol separated from the blood due to the soluble nature of the assemblies.
The molecules have specific receptors that direct them to certain tissues in the body, and there are several types of lipoproteins based on their density. For example, high density lipoprotein (HDL) is quite dense and used to transport cholesterol from body tissues to the liver. Low density lipoprotein (LDL) is slightly less denser and used to transport cholesterol from the liver to the cells of the body.
These different molecules are measureable and make up what we evaluate when we draw blood to check a person's "lipid panel or profile." Two of the most important measureable varieties (of cholesterol?) include LDL "Bad Cholesterol" and HDL "Good Cholesterol." These canbe further broken down into what are known as sub-particles that include either the very small and dense or the large and more buoyant varieties. This breakdown is highly predictive of the development of atherosclerotic plaque in vessel walls which is the predominant lesion in the development of atherosclerosis (the cause of coronary and peripheral artery disease). The smaller and denser the LDL, the greater chance of developing plaques. The smaller and denser the HDL, the less effective it is in removing cholesterol from the blood and returning it to the liver.