1) Class of lipoproteins that promote transport of cholesterol from extrahepatic tissue to the liver for excretion in the bile; synthesised by the liver as particles lacking a lipid core, they accumulate a core of cholesterol esters during reverse cholesterol transport and transfer them to the liver directly or indirectly via other lipoprotein; HDL also shuttle apolipoproteins C-II and E to and from triglyceride-rich lipoproteins during catabolism of the lipoproteins; serum HDL cholesterol has been negatively correlated with premature coronary heart disease.

2) High density lipoproteins are the smallest and densest lipoproteins, and contain a high proportion of protein. They are synthesised in the liver as empty proteins and they pick up cholesterol and increase in size as they circulate through the bloodstream. Because HDL can remove cholesterol from the arteries, and transport it back to the liver for excretion, they are seen as "good" lipoproteins. (from Wikipedia)