Enzyme cooperativity is a trait displayed by enzymes which have multiple binding sites that modify one another's effectiveness. This happens because the enzyme's chemical structure is subtly changed as the binding sites are utilized. An enzyme demonstrating positive cooperativity will have increased affinity for the substrate molecule when one or more of its binding sites are already attached. An enzyme with negative cooperativity will lose affinity as its binding sites are filled. Both of these mechanisms allow chemical pathways within the body to be fine-tuned.
A good example of positive cooperativity is the binding of oxygen to hemoglobin. A hemoglobin molecule is made up of four subunits, each of which can bind one oxygen molecule. However all the bonds are not equal; once one of the hemes has attached to an oxygen, the affinity of the other three active sites increases tremendously - the oxygen affinity of a hemoglobin with three sites filled is about 300 times greater that that of a hemoglobin with all four sites open.
Glutamate dehydrogenase is an example of an enzyme that has negative cooperativity. This article has excellent illustrations that show how the protein structure of the enzyme changes as the binding sites are filled.