Substance derived from silica (sand is a silica). The unique fluid properties of silicone give it a great deal of slip, and in its various forms it can feel like silk on skin, impart emollience, and be a water-binding agent that holds up well, even when skin becomes wet. 
There are numerous forms of silicones used in cosmetic products, particularly leave-on skincare products and all manner of hair-care products. Common forms of silicone are cyclopentasiloxane and cyclohexasiloxane; other forms include various types of dimethicone and phenyl trimethicone.
Claims that silicones in any form cause or worsen skin concerns have not been substantiated in published research, nor have reports that silicones are sensitizing to or “suffocate” skin. Almost all of these claims are either myths or based on anecdotal evidence, which isn’t the best way to determine the safety or efficacy of any cosmetic ingredient. How do we know that silicones don’t suffocate skin? Because of their molecular properties they are at the same time porous and resistant to air. Think of silicones in a skincare formula like the covering of a tea bag. When you steep the tea bag in water the tea and all of its antioxidant properties are released.
Silicones remain on the surface of your skin and the other ingredients it’s mixed with “steep” through. All ingredients must be suspended in some base formula; some of the ingredients remain on the surface, some are absorbed. The intent is for the “actives” to get through.
The unique molecular structure of silicones (large molecules with wide spaces between each molecule) allows them to form a permeable barrier and also explains why silicones rarely feel heavy or occlusive, although they offer protection against moisture loss. 
Interestingly, silicone has been shown to be helpful for offsetting dryness and flaking from common ingredients such as benzoyl peroxide. 
- Draelos Z, Callender V, Young C, Dhawan S. The effect of vehicle formulation on acne medication tolerability. Cutis. 2008;82(4):281-4.
- Fabbrocini G, Annunziata M, D’Arco V, De Vita V, Lodi G, Mauriello M, Pastore F, Monfrecola G. Acne Scars: Pathogenesis; Classification and Treatment. Dermatol Res Pract. 2010;2010:893080.