Crushed fossillized diatoms. Mostly Silicon Dioxide (Silica). Food grade is uncooked amorphous silica and not a silicosis hazard. Non-food grade like that used for pool filters has been cooked into crystallized Silica which is a silicosis hazard.
It is a desiccant, and thus can kill small insects by drying them out. Completely harmless to ingest, but counter to some of the myths, it is not useful to ingest. I use it in my coop to dry out poop, and since dry poop does not smell as much as wet poop, it tends to reduce the smell in the coop.
I get mine for $.99 a pound out of a bulk garbage can at my feed store, I expect a 50lb bag would be about $30. I saw a 2.5 lb bag at a vendor at a dog show for like $15, so don't get ripped off.
