Lipids are broadly defined as any fat-soluble (lipophilic), naturally-occurring molecules, such as fats, oils, waxes, cholesterol, sterols, fat-soluble vitamins (such as vitamin A, vitamin D, vitamin E and vitamin K), monoglycerides, diglycerides, phospholipids, and others.

The main biological functions of lipids include energy storage, acting as structural components of cell membranes, and participating as important signaling molecules.

Although the term lipid is sometimes used as a synonym for fat, fats are actually a subgroup of lipids called triglycerides and should not be confused with the term fatty acid. Lipids also encompass molecules such as fatty acids and their derivatives (including tri-, di-, and monoglycerides and phospholipids), as well as other sterol-containing metabolites such as cholesterol.

Lipids are a diverse group of compounds that have many biological functions, such as acting as structural components of cell membranes, serving as energy storage sources and participating in signaling pathways.

References

Fahy E, Subramaniam S, Brown HA, et al (2005). "A comprehensive classification system for lipids". J. Lipid Res. 46 (5): 839–61. PMID 15722563.?