What is the difference between HDL (high-density lipoprotein) and LDL (low-density lipoprotein)?

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.
HDL good; LDL bad. If cholesterol is garbage, then HDL is in the garbage trucks( being carried away from the cells); LDL is in the delivery trucks ( going to the cells).
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.