Diatomaceous Earth is a fossilized algae composed of diatoms, it's mined and ground into a fine powder. DE is used for a great many things, almost all of which fall into two main (and rather unrelated) categories: health benefits and pest control. It is either taken internally as a supplement or used as a natural solution to treat all kinds of insects. There are other grades like pool grade DE that are used to filter water, but have been treated and are not safe for consumption. Make sure you have the right grade for the job.