LDL is a low density lipoprotein. It starts it s life as a very low density lipoprotein (VLDL) which transports fat in the form of triglycerides and cholesterol around the body. As it loses the triglycerides to cells for energy or storage, it becomes an LDL, containing mainly cholesterol. Cholesterol is a vital substance within the body, so the purpose of LDL is to get the cholesterol to cells that need it. For the cholesterol to be transferred, a cell needs a specific receptor. Without this receptor there will be no transfer of cholesterol, so it s impossible for cholesterol to just clog up your arteries.. If you produce too many VLDLs, it will result in high levels of LDL as cells only have the receptor when they need cholesterol. Remember that unless cells have the receptor, the LDL cannot unload it s cargo. A HDL is a high density lipoprotein. One of it s functions is to return used cholesterol, from cells back to the liver. This is a cyclic process. LDLs supply the cells with fresh cholesterol and HDLs remove the used cholesterol. Neither is good or bad, they both play an important role, without which we would simply die. Another purpose of HDL is to hand out and take back things called appolipoproteins to VLDLs. These appolipoproteins are what activates the enzymes in your capillaries to transfer the triglycerides from the VLDLs to cells