A chemical reaction is an equation that shows what happens in a reaction. All chemical reactions are composed of reactants (chemicals that are present before the reaction) and products (chemicals that are left after the reaction takes place.

Na⁺ + Cl^- ==> NaCl

In the chemical equation above, the Na⁺ and the Cl^- are the reactants. The NaCl is the product. The symbol between the Cl^- and the NaCl, (==>), is the yields sign. The arrow points towards the products and is used to show how the reaction takes place.

Information provided by : http://library.thinkquest.org