Enzymes are proteins that catalyze (i.e. accelerate) chemical reactions. http://en.wikipedia.org/wiki/Enzyme

Control all of your bodily functions through the reactions they create.

Wikipedia always give good explanations! It would be better then me trying to explain it anyway! http://en.wikipedia.org/wiki/Enzyme

Enzymes are "biological catalysts." "Biological" means the substance in question is produced or is derived from some living organism. "Catalyst" denotes a substance that has the ability to increase the rate of a chemical reaction, and is not changed or destroyed by the chemical reaction that it accelerates. Generally speaking, catalysts are specific in nature as to the type of reaction they can catalyze. Enzymes, as a subclass of catalysts, are very specific in nature. Each enzyme can act to catalyze only very select chemical reactions and only with very select substances. An enzyme has been described as a "key" which can "unlock" complex compounds. An enzyme, as the key, must have a certain structure or multi-dimensional shape that matches a specific section of the "substrate" (a substrate is the compound or substance which undergoes the change). Once these two components come together, certain chemical bonds within the substrate molecule change much as a lock is released, and just like the key in this illustration, the enzyme is free to execute its duty once again. Many chemical reactions do proceed but at such a slow rate that their progress would seem to be imperceptible at normally encountered environmental temperature. Consider for example, the oxidation of glucose or other sugars to useable energy by animals and plants. For a living organism to derive heat and other energy from sugar, the sugar must be oxidized (combined with oxygen) or metabolically "burned" However, in a living system, the oxidation of sugar must meet an additional condition; that oxidation of sugar must proceed essentially at normal body temperature. Obviously, sugar surrounded by sufficient oxygen would not oxidize very rapidly at this temperature. In conjunction with a series of enzymes created by the living organism, however, this reaction does proceed quite rapidly at temperatures up to 100°F (38°C). Therefore, enzymes allow the living organism to make use of the potential energy contained in sugar and other food substances. Enzymes or biological catalysts allow reactions that are necessary to sustain life proceed relatively quickly at the normal environmental temperatures. Enzymes often increase the rate of a chemical reaction between 10 and 20 million times what the speed of reaction would be when left uncatalyzed (at a given temperature). Nutrients locked in certain organics are complex macromolecules, or in hard-to-digest matrices may be released or predigested by a high degree of heat or concentrated acid treatment. In an alternative manner, specific enzymes can promote the pre-digestion of certain complex nutrients and facilitate the release of highly digestible nutrients in organics during processing without the need of excessive heat or rigorous chemical treatment. http://www.microtack.com/html/enzyme1.htm