Simple lipids are those which contain esters of fatty acids and alcohol. They are further classified into fats and waxes. The alcohol in fats is glycerol, and the alcohol in waxes is anything other than glycerol. This alcohol has a higher molecular weight.

Neutral lipid, fat, triacylglycerol, or triglycerides are esters of glycerol with fatty acids. Triacylglycerols are the most abundant group of lipids that primarily function as fuel reserves of animals. The fat reserve of normal humans is sufficient to meet the body caloric requirements for two to three months. Fat primarily occurs in adipose tissue: Adipocytes of adipose tissue predominantly found in the subcutaneous layer and in the abdominal cavity are specialised for storage of triacylglycerols.