Silicones are a class of polymers based on an alternating chain of silicon and oxygen atoms, rather than a chain of carbon atoms as in most polymers. Different organic groups are attached to this silicon-oxygen backbone to produce a vast array of silicones with widely differing properties. Silicones can be liquid or a soft solid similar to rubber. There are many formulations for many purposes; not all silicone is the same. Not all types are suitable for contact with human skin, but some types are bio-compatible and are even used in implants. They should not be confused with the element 'silicon' (no "e").
As solids, silicones are very tactile, being smooth and silky to the touch. They do not feel clammy, as they warm quickly to the surrounding temperature. Most solid silicones are also flexible and malleable so that they always feel comfortable, retaining heat well.
Silicones are generally regarded as the best materials from which to make dildos and other insertive sex toys. They are tough, are easy to clean (you can even put most of them on the top shelf of a dishwasher) and non-porous if well made, so no bodily fluids can seep in and stain or taint the product. Silicone can also be manufacturered and dyed in a myriad of colours, so a dildo collection doesn't have to be universally black.