Kieselguhr or diatomite, is technically a clay, formed from the fossilized shells of microscopic unicellular aquatic plants (i.e., phytoplankton). Kieselguhr has a myriad of uses (see more at diatomaceous earth), from abrasive cleansers (including toothpaste and industrial steel polish), to dynamite. Sometimes sold under the trade name "Dimatos," kieselguhr's usefulness in dynamite manufacture comes from its ability to absorb an impressive three times its weight in nitroglycerin.
Kieselguhr's porous nature and natural, easy cultivation also makes it extremely useful in filtration, and it is safe for use in some food preparation capacities, such as brewing, water purifying, and clarification of viscous potable liquids like sugared syrups.
Since the term kieselguhr is entirely synonymous with diatomaceous earth, I will not duplicate the information that can be found at that node, but I will add that diatomaceous earth seems to be the preferred American term, while kieselguhr is what it's called everywhere else, including Britain.
Sources:
http://www.ibiblio.org/herbmed/eclectic/bpc1911/kieselguhr.html
http://www.tiscali.co.uk/reference/dictionaries/difficultwords/data/d0007330.html
http://cgi.peak.org/~jeremy/retort.cgi?British=kieselguhr
(Credit also the Connections TV series, hosted by James Burke, who made recurring mention of kieselguhr throughout the series).