A lipid is a type of organic molecule that is insoluble in water but soluble in nonpolar solvents. Lipids are commonly known as fats, oils or waxes and they are an important part of the diet for humans and other animals. They serve various functions such as energy storage, cell membrane structure, and signaling molecules among others. Examples of lipids include triglycerides (fats), phospholipids (which make up cell membranes) and sterols (such as cholesterol).