Anyways, google is ur friend. JB Weld is something that reacts to create a weld-type of bond, usually between two metals. It basically creates a "weld" (which I'm assuming you know is) by mixing two chemicals and allowing them to cure.
JB weld is an epoxy based adhesive. When you mix the 2 parts, a chemical reaction takes place and it'll form a tempory bond. This stuff is ok, not the greatest. Also you need to give it about 24 hours for it to fully cure. You can pick it up at just about any auto parts store. And where exactly is this crack? Do you have pics for us?