An allosteric enzyme is one in which the activity of the enzyme can be controlled by the biniding of a molecule to the "allosteric site". This really just means somewhere other than the active site. Thus allosteric control of an enzyme can be classed in two ways. A positive allosteric modification is the binding of a molecule to the enzyme which increase the rate of reaction. Sort of like catalysing the catalysing effect of an enzyme. Obviously the opposite is true of negative allosteric modification. A good example for this is the activity of phosphofructokinase, which is promoted by a high AMP concentration, and inhibited by a high ATP concentration. This should make sense if you think about the action of a kinase etc.
