Enzymes are biological catalysts, and they are very common in nature. In fact, all living cells need enzymes to continue their function. Without enzymes, chemical reaction in cells would not be fast enough to keep the balance of life.
Enzymes are keyed to react only with certain Substrates, or chemical reactants. This way, they do not cause uncontrolled reaction in every cell. Like inorganic catalysts, enzymes change the reaction rate of two or more chemical by altering the electrical charge between the two reactants. As the reactants trade molecules or atoms, the enzyme remains unchanged, since it is usually larger than the reactants themselves.
An Unstable Intermediate is the condition of a reactant, or substrate, after bonding with the enzyme. Since each reaction needs energy to complete, the substrate can be strained by the necessary expenditure and be in danger of losing its shape and coherence. The enzyme, by bonding and forming the unstable intermediate, also allows the reaction to occur with less energy usage, and so the substrate is able to return to its stable form each time with less danger. Enzymes both speed up, slow down, and alter the rate and type of reactions, so this is only one of the functions they serve.