Enzymes are three dimensional proteins that combine with reactants of a chemical reaction in order to lower the amount of energy needed for the reaction to take place. Reactants of a chemical that are acted on by an enzyme are known as the substrate.
Inhibitors are molecules that attach themselves to enzymes and disrupt the ability to speed up a chemical reaction. Noncompetitive inhibition occurs when a molecule disrupts the shape of an enzyme by binding somewhere other than the active site. The active site is the area of an enzyme where a substrate typically attaches.
Competitive inhibition occurs when a inhibitor molecule has a shape that is very similar to the substrate. Enzymes are unable to distinguish the inhibitor from the normal substrate.