Surfactant    (aka Surface active agent)
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An important component in nearly all shampoos and cleansers. Used to help wet the surface better. They are also used to help remove dirt and oils from the surface to be cleaned. Keeps dirt and oil suspended in water so they can be rinsed away. The surfactant is able to get oil and oily dirt to rinse away with water because each molecule of the surfactant has a water-loving (hydrophillic) head, and a water-hating (hydrophobic) tail. Oils are attracted to the tail, and water is attracted to the head. Therefore, the surfactant molecules are able to grab the oil with one end, and the water with the other, so that the oil can be washed down the drain with the water.

Some surfactants are used as emulsifiers. Since they have both an oil-loving end and a water-loving end, they are able to hold oil and water together in a product, so the oil and water components don't separate.

There are four major types of surfactants: Cationic, Anionic, Nonionic, and Amphoteric:
Cationic surfactants: These have a positive charge. Often used as an antistatic or for sanitizing. Used as friction reducers in conditioners and fabric softeners.
Anionic surfactants: These have a negative charge. They are strong cleansers—often used as stain removers in household detergents.
Nonionic surfactants: These don't have an electrical charge. They aren't effected by hard water, and dissolve oil and grease well, so are often used as oven cleansers to remove baked on grease.
Amphoteric surfactants: Can be negatively or positively charged depending on the situation. These are frequently used in cosmetics and shampoos because they are very mild [Winter pgs 502-503].
For more information, see Surfactants and How surfactants work.
See also: Ion Cation Anion Amphoteric Nonionic
Source(s): Winter http://www.scienceinthebox.com/en_UK/glossary/surfactants_en.html