Very little cholesterol is present in the body as a free lipid.

In the blood most of it is bound to two kinds of lipoprotein particles, HDL and LDL. Because of this measurements of total blood cholesterol are not currently regarded as helpful.

Cholesterol in HDL is sometimes informally called "good cholesterol". That is because HDL is responsible for a process called reverse cholesterol transport. That means that, through the action of a protein called SR-BI, it can be taken up by the liver and its cholesterol disposed of. SR-BI is also important in platelets. There is a free-access review of this here SR-BI as target in atherosclerosis and cardiovascular disease - A comprehensive appraisal of the cellular functions of SR-BI in physiology and dise... - PubMed - NCBI.

Cholesterol in LDL is sometimes informally called "bad cholesterol". Elevated LDL levels are now regarded as a primary risk factor for cardiovascular disease rather than total blood cholesterol levels.

This is because, through complex mechanisms, LDL can adhere to blood vessels which, through a cumulative process (formation of a plaque) can block them. If this happens in heart blood vessels it leads to a myocardial infarct. If a plaque of LDL in a blood vessel breaks free it can enter the brain, blocking brain blood vessels - this leads to an ischaemic stroke. There is a free-access review of this here A review of low-density lipoprotein cholesterol, treatment strategies, and its impact on cardiovascular disease morbidity and mortality. - PubMed - NCBI.

The roles of HDL and LDL in cardiovascular disease are fairly well-understood though we still have more to learn.