Bentonite is a type of volcanic ash which looks like little credit cards under a microscope. The edges are positively charged, while the face and back are negative. This gives it the ability to pick up a lot of positively charged particles (ions).
Bentonite is effectively used in cleaning body waste (colonic irrigation), since many of the body's toxins are positively charged.
It is believed that Bentonite doesn't have a chemical effect on the body, only physical (the attraction to positive ions).
Info from:
Public Research Project: Bentonite / An Educational Compilation / Compiled by Jason R. Eaton and Tammy M. Eaton
July 9, 1995 / http://members.nbci.com/windfire/bent.html