Many chemical reactions do not go to completion. That is to say when the reactants are mixed and the chemical reaction proceeds it only goes to a certain extent, and at the end there are measurable amounts of both reactants and products present in the system. A system is then said to be at equilibrium.
A measure of how far a reaction goes is given by its "equilibrium constant" which can have very small or very large values.
This section introduces you to the language used for these reactions, and introduces you to some important classes.