Its an emulsion of silicone polymers. My company used it in a reagent for biotech assays for a while, before we fixed our foaming problem with changes to the engineering of the wash stations. Its non-toxic, but will probably screw up head retention unless you do what the big breweries do: filter your beer before bottling. Here's a link to a fact sheet from a manufacturer:
http://www.baschem.co.uk/downloads/fd20p.pdf
And a pic of the chemical structure
Its essentially quartz (silicon dioxide) where some of the oxygen atoms in the crystal lattice have been replaced by methyl groups (a carbon and 3 hydrogens), which turns it into a linear molecule that is sparingly soluble. This structure makes it like to hang out at the air/water interface where it disrupts surface tension.