Enzymes are protein compounds created by ribosomes from a cell's genetic code, copied from DNA to RNA, and finally translated into its final form. Enzymes are organic catalysts; they promote certain chemical reactions without actually being consumed by those reactions. They do this by acting as a substrate for an intermediate step in a chemical reaction, lowering the energy required to get the reaction started.
Enzymes are absolutely crucial to life on Earth. This class of compounds is involved in innumerable chemical activities in the cell. Many of the reactions that take place in cells do not occur spontaneously at the temperatures within cells; cells only need certain reactions to occur at certain times. When they do, they use an enzyme to do it. The enzyme makes the reaction happen relatively easily, exactly when the cell needs it to occur.
Enzymes are proteins, just like the structural proteins that make up many of the structures within cells. However, they tend to be smaller than structural proteins. Because enzymes are not used up in the reactions they catalyze, they can be used over and over again. Indeed, to prevent existing enzymes from working when the cell does not need them, cells must sometimes produce other compounds to shut the enzymes off.