Polydimethylsiloxane, also referred to as dimethicone, is the most common type of silicone.  Its structure is a linear chain of silicon-oxygen as a backbone with methyl groups attached to each silicon.  The backbone is very flexible, and longer chains can become entangled, giving the molecule a very high viscoelasticity.  This means that it acts as a liquid over long periods of time and as a rubbery solid over short periods of time.  This property makes it ideal as a sealant or adhesive, as over time it will fill in surface imperfections and gaps.
 
 

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      It is stable at high temperatures, resistant to water, chemicals, and oxidation, and does not conduct electricity.  It is used for adhesives, caulks, sealants, and lubricants, and is a component of defoaming agents, heat transfer fluids, and damping fluids, as well as many popular cosmetics and conditioners.  It was once used in breast implants, but use has decreased due to safety concerns.

References:

Silicone: http://en.wikipedia.org/wiki/Silicone

Polydimethylsiloxane: http://en.wikipedia.org/wiki/Polydimethylsiloxane

Sealant available at: http://www.homedepot.com/prel80/HDUS/EN_US/diy_main/pg_diy.jsp?prod_id=101088

The many uses: http://householdproducts.nlm.nih.gov/cgi-bin/household/brands?tbl=chem&id=106&query=dimethicone+350&prodcat=all