The left diagram shows a section from a typical silicone molecule.

It is essentially an organically modified quartz, i.e. two oxygen atoms attached to every silicon atom have been replaced by methyl groups. This changes the structure from the three-dimensional backbone of quartz to linear polymer molecules.
Silicones therefore are composed of the elements Si, O, C and H. Silicone molecules are usually tangled up in each other. However, the methyl groups are free to rotate about the -Si-O-Si- chain (see also the video as wmv or mov).

Other groups can be attached to the silicon-oxygen backbone instead of methyl groups.
The linear silicone polymers can be crosslinked to each other to different extents, i.e. covalently via groups of atoms. This explains the various different properties of the corresponding silicones (silicone resin, silicone rubber).