LDL and HDL are forms of what?

cholesterol

Low-density lipoprotein (LDL) and high-density lipoprotein (HDL) are each one of the five major groups of lipoproteins, referring to markers for heart disease.

Higher levels of LDL particles promote health problems and cardiovascular disease, they are often called the bad cholesterol particles, (as opposed to HDL particles, which are frequently referred to as good cholesterol or healthy cholesterol particles).