Cab-o-sil is a brand name. It's powdered silica. Other companies sell the same stuff under different names. It's great for a gluing thickner. It hardens like a rock. You won't enjoy sanding it. Don't forget, you can thicken epoxy with finde wood dust from your finishing sander or blend these thickners.

Along with the booklets mentioned by the others, the Gaugeoun. Bros. (West System Brand) has an excellent video on epoxy use. I believe it about $15. Shows everything you could possibly do with it.