Enzymes are catalysts. They catalyze a large number of reactions in our body and in our industries, thus enabling the production a large number of useful chemicals. Enzymes are typically protein molecules and are necessary for sustaining life on this planet. The metabolic reactions are simply not fast enough on their own. Enzymes catalyze these reactions and increase the reaction rates to levels required to sustain life.
Enzymes are very specific in their activity and one enzyme will only catalyze one reaction (or so). Enzymes function by providing reaction pathways with lower activation energy (energy required to start a reaction) and thus increase the rate of reactions. It is important to note that the final energy changes would be the same as without enzyme action, it is just faster with the help of enzymes.
Hope this helps.