In physiology, LDL cholesterol is "cholesterol which is contained in or bound to low density lipoproteins (LDL), including cholesterol esters and free cholesterol." Laymen commonly call it "bad cholesterol", but this is misleading; cholesterol itself is a single compound without good or evil aspects. LDL, however, is carried, in the blood, to cells where the carried cholesterol is metabolized, or, most significantly, to blood vessels where it is deposited.
Most atherosclerosis is caused by cholesterol deposited by LDL. In contrast, HDL cholesterol is considered "good" because it returns cholesterol to the liver from blood vessels. A sufficient level of HDLs will scavenge LDL and prevent atherosclerosis.
LDL can be reduced by a variety of drugs such as statins.