Enzymes are biological catalysts. The job of an enzyme is, generally, to speed up a chemical reaction. Enzymes do this by providing a lower energy route for a reaction, so the reaction can occur at a lower temperature, but the reaction speed can be fast.
Enzymes are complex proteins, and they are very specific about the temperatures at which they work. At temperatures cooler than this, the enzyme will work slowly. Much above the optimum, and the protein will change shape, and will stop working permanently. This is called denaturing. pH (acidity) is also a contributing factor, and this has an absolute optimum value as well.
Enzymes are specific to a single reaction. This reaction takes place on a special active site on the protein.
Enzymes are said to work by 'lock-and-key' theory. Some enzymes collect two or more reactant 'keys', and join them together, while others do the reverse. Chemicals changed by an enzyme are called substrates.
Our bodies contain hundreds of thousands of different enzymes, in every living cell of our bodies. We need enzymes to live as we do - life would be really impractical if we had no enzymes to help us.
Finally, it should be mentioned that enzymes are able to be released at given quantities, giving good control over reaction speeds in the body.