When molecules react in order to form different molecules, a chemical reaction takes place. Chemical reactions can be represented by a chemical equation. The chemicals at the start of the reaction are known as the reactants and the chemicals produced by a reaction are known as the products. In a chemical equation the reactants are found at the left side of the the equation. An arrow is used to represent the occurrence of the chemical reaction. The chemicals found to the right of the reaction arrow are known as the products.
In this image, hydrogen and oxygen react in a chemical reaction forming water. Hydrogen and oxygen are the reactants. The product, water, is being formed by the chemical reaction.