What Is It?
Cyclomethicone is a general term describing both individual and/or a mixture of cyclic siloxane materials that have many uses and are found in a variety of consumer products. These materials are derived from the natural elements silicon and oxygen.  
Why is it used in cosmetics and personal care products?
D5 and D6 are the predominant cyclomethicones used in cosmetics and personal care products, due to their excellent skin and hair conditioning properties.  Cyclomethicone D4 is rarely used as a cosmetic ingredient. D4 can also be present at very low residual levels (generally <0.1%) in D5 and D6, since it is used as a starting material in the production of those cyclomethicones. During this manufacturing process, virtually all of the material is consumed leaving only a tiny amount of D4.
Cyclomethicone ingredients are used in cosmetic products where eventual complete evaporation of the siloxane carrier fluid is desired. In this way they are useful for products like deodorants and antiperspirants that need to coat the skin but not remain tacky afterward. They provide other important product performance benefits such as facilitating a smooth texture and an even application.  Thus, they may also be found in sunscreens, shampoos, conditioners, moisturizers, lotions, etc.