Hide glue is a thick yellow glue made from boiling
animal hides, though now I think it's mostly synthetic. It is
mainly used as a woodworking glue and can be found in any
hardware or craft store. It provides the carbon component for
the eutectic bond created by fusing. You can use saliva as your
source of carbon but the hide glue and water mixture helps hold
the granules in place, so it's a little more practical.
Someone also asked earlier about fusing chips and dust. The
chips are fused the same way as the granules, either by plating
them with copper or feeding a copper/glue solution and then
heating. The dust is done in layers. You add a tiny drop of
water to the dust to clump it together and then you pack it into
the spaces you've created on your piece (usually by fusing wire
or chips, etc.). You then feed in the copper solution--the dust
draws it down to the base of the piece by capillary action--and
then fire that. Since the first layer generally looks somewhat
melted, you usually add another layer. You can add even more
layers, and build it as high as you want.
Hope that helps!