Fats consist of a wide group of compounds that are generally soluble in organic solvents and generally insoluble in water. All lipids (fats) consist of two or three parts: a glycerol part and at least two hydrocarbon tails. Some have a phosphate group instead of a third tail, and are called phospholipids. There are two main types of lipids: saturated and unsaturated. Saturated lipids have long carbon-carbon single-bonded tails saturated with hydrogen atoms. The carbon tails of unsaturated lipids are not completely saturated with hydrogen, and thus have some double or triple bonds between carbons. The physical manifestation of this is that saturated lipids tend to be solid at room temperature, and unsaturated lipids tend to be liquids at room temperature. For more information see wikipedia:Fat.