1. Reactants: These are the substances that are present at the beginning of the reaction and are going to be changed during the reaction. They are written on the left-hand side of the equation.

2. Products: These are the substances that are formed as a result of the chemical reaction. They are written on the right-hand side of the equation.

Here's a simple example:

Reactants → Products

2H₂ + O₂ → 2H₂O

* Reactants: Hydrogen gas (H₂) and Oxygen gas (O₂)

* Products: Water (H₂O)