Big difference. Ldl, when found in high concentrations in the blood, can enter your artery walls and initiate an inflammatory reaction that culminates in the development of atherosclerosis, plaque formation in the arterial wall. Hdl had many functions. If HDL is functioning well it can help to prevent atherosclerosis. One of its major functions is to remove cholesterol from the arterial wall.
Clarification Issues. Lipoproteins transport all fat molecules within the water outside cells. Cholesterol is a fat molecule within all animal cell membranes. Cholesterol is not lipoprotein; lipoproteins carry cholesterol molecules along with all other fat molecules. Ldl particles (containing whatever fats) drive atherosclerosis if not at low concentrations. Hdl particles can remove excess fat from plaque.