DKM: DE is (from Wikipedia) "Diatomaceous earth consists of fossilized remains of diatoms, a type of hard-shelled algae. It is used as a filtration aid, as a mild abrasive, as a mechanical insecticide, as an absorbent for liquids, as cat litter..."
FOOD GRADE DE (NOT the stuff for pool filtration, which has been chemically treated) is a 'natural' alternative for providing PROPHYLACTIC (preventative) pest control in animals. It has the texture of a fine sand or powder and the hard "crystalline" edges cut into parasites/worms and mites/lice, causing these pests to dehydrate and die off. It can be added to feed and water, or animals can be dusted with it, as can walls and bedding in housing. Animals who injest it are also excreting it, so 'droppings' contain DE, which also purportedly help to reduce the fly population. When used in bedding, it helps to dry bedding out and thus keep down odor and pests which might use the bedding to breed.
I'm getting my first flock in a few months, so I've never used it in chickens, but my dog had a chronic flea problem we couldn't resolve and DE cleared it up in four weeks. I ordered a 10-lb bag off
Amazon, but my feed store is going to start carrying it for me in 50-lb bags, which is nice of them and will save me a few bucks on S&H.
Here's a link I found really helpful:
http://wolfcreekranch1.tripod.com/defaq.html
ETA: IF you use this and your critters develop an obvious problem with worms, mites/lice, etc., it is IMPORTANT to treat with a proven solution (i.e. medication from
Tractor Supply, etc.). DE is thought to be very useful in PREVENTING these problems, but is NOT A CURE.