It's diatomaceous earth. Essentially it's skeletons of microscopic creatures that appears as a white powder. It's the critical element that allows DE filters to function. DE filters are the finest filters sold for pools, filtering down to something like 2-5 microns. Cartridge filters filter to 20 microns, and sand filters to about 50-60 microns. Some of the most expensive robotic cleaners (Aquabot, Blue Diamond, Dolphin Dynamic) can match the DE filter at the 2-5 micron range, but they aren't daily filters.

But we also can add a little DE to sand filters to get the water a little clearer. I was one of the first to experiment with this--Poconos led the way. It's very inexpensive to do: I've been using one 10 # box for 2 seasons, and am about to open the 2nd 5# bag in the box--and the 10# cost me $8. The results, combined with a few other strategies, have gotten my water from very clear to crystal, crystal clear.

DE also is excellent for ant control. Apparently, it's quite deadly to them--something about the sharp edges on the powder grains I think.