Sterols are structural lipids present in the membranes of most eukaryotic cells. Their characteristic structure is the steroid nucleus (in blue) consisting in four fused rings (A,B,C,D), three with six carbons (A,B,C) and one with five (D). The steroid nucleus is almost planar, and relatively rigid; the fused rings do not allow rotation about C-C bonds.

Cholesterol is the major sterol in animal tissues. Its structure is represented below.

Cholesterol is amphipathic, with a polar head group (the hydroxyl group at C-3, in red) and a nonpolar hydrocarbon body (the steroid nucleus, in blue, and the hydrocarbon side chain at C-17, in green).

The sterols serve as precursor for a variety of products with specific biological activities. Bile acids, in which the side chain in C-17 is hydrophilic, act as detergents in the intestine, emulsifying dietary fats to make them more readily accessible to digestive lipases. A variety of steroid hormones are also produced from cholesterol by oxidation of the side chain at C-17.