Wikipedia describes silicones as "Silicones are largely inert, man-made compounds with a wide variety of forms and uses. Typically heat-resistant, nonstick, and rubberlike, they are commonly used in cookware, medical applications, sealants, adhesives, lubricants, and insulation." 

Now you have to wonder why a sealant, insulator, and adhesive is good for your hair.  The answer to that question is that it is not.  What it does is form a sealing film over the surface of every single hair shaft.  It keeps moisture and oil out of the hair while allowing the hair to continue to loose moisture and oil. 

This eventually results in dry brittle hair that breaks and becomes damaged.  In my humble opinion, these things are poison for your hair.  My experience with them has been that they initially boost shine and reduce frizz but that over time, they actually increase the damage to the hair.  They are difficult to get rid of.  It requires a sulfate based shampoo to remove them. 

Here is a sample list of silicones that are commonly found in products.

Antifoaming agent - Dimethicone, Cetyl Dimethicone
Corrosion inhibitor - Stearamidopropyl Dimethicone
Film former - Stearamidopropyl Dimethicone
Hair conditioning agent - Amino Bispropyl Dimethicone, Aminopropyl Dimethicone, Amodimethicone, Amodimethicone Hydroxystearate, Dimethoxysilyl Ethylenediaminopropyl Dimethicone, Hydroxypropyldimethicone
Skin conditioning agent - emollient - Behenoxy Dimethicone, C24-28 Alkyl Methicone, Cetyl Dimethicone, Hexyl Methicone, Stearoxy Dimethicone
Skin conditioning agent - miscellaneous - Hydroxypropyldimethicone
Skin conditioning agent - occlusive - Methicone, Dimethicone, C30-45 Alkyl Dimethicone, C30-45 Alkyl Methicone, Cetearyl Methicone, Cetyl Dimethicone, Stearyl Dimethicone, Stearyl Methicone
Skin protectant - Dimethicone
Surface modifier - Methicone
Viscosity increasing agent - nonaqueous - C24-28 Alkyl Methicone, C30-45 Alkyl Methicone

References - CosmeticsInfo.org and Wikipedia