An enzyme is a protein that makes a chemical reaction happen faster. The human body oversees thousands of chemical reactions in a given day. Some of these reactions would not ever happen - or would take much longer to occur - if not for enzymes that assist the chemical reaction to occur quickly. (Some enzymes increase the rate of a chemical reaction by one million times.)
Because our bodies need certain substances to live (like ATP, the energy "currency" of the body), we must be able to have a quick and reliable way to produce these substances. Enzymes ensure that we have the material we need at the correct time to make our bodies work, move and function properly.
Enzymes are specific for the chemical reaction that they regulate. Enzymes are recycled as well: as soon as an enzyme assists in a chemical reaction, it is then ready to assist in making the same reaction occur again. Enzymes are regulated as well, as our bodies may require different amounts of material at different times.
For instance, let's use Substance C as an example of a material that our body needs. And, let's further say that Substance C is made from Substance A and Substance B combining in a chemical reaction that is catalyzed by Enzyme Z.
If our body suddenly needed more of Substance C, it could increase the number of Enzyme Z molecules, which would be available to convert more of Substance A and Substance B into Substance C. Alternatively, our body could reduce or inhibit Enzyme Z so that our body would make less Substance C in a situation where less Substance C is required. In this way, our body maintains efficiency.
Enzymes are biological catalysts. They catalyze reactions, that is to say, increase the rate of reactions. Enzymes are generally proteins and they catalyze thousands of reactions, making life possible. They act by lowering the activation energy of a reaction (by providing an alternate reaction pathway or stabilizing transition state or destabilizing reactant ground stage) and thus promote conversion of reactants to products. They can be very specific (i.e., catalyze only one specific reaction) and their activity can be affected by inhibitors and activators. This specificity is explained by 'lock and key' or 'induced fit' model, both of which state (in essence) that enzymes attach to specific sites on the surface of substrates (or reactants) in order to catalyze their conversion to products.
Metabolizing is a great example of an application of enzymes in organisms. They can also be used in industries as catalysts.