An enzyme is a highly selective biological catalyst which accelerates specific biochemical reactions, speeding up the reactions without being 'used up'. A particular enzyme will only work on a particular substrate, when the substrate binds to the enzymes active site.
Under appropriate reaction conditions (e.g. temperature, pH, cofactors, etc), the substrate binds to the active site of the enzyme. This begins the chemical reaction, which is greatly accelerated by the enzymes activity resulting in a new product or molecule that then separates from the enzyme. The enzyme can then go on to catalyze other substrates.
Enzymes significantly speed up the rate of reaction that take place within cells.
Enzymes are vital for life. They facilitate cell function by speeding up reactions within the cell.