A typical chemical usually results in an equilibrium system. Called a dynamic equilibrium, this is when the rate of change of the forward and reverse reactions occurs at the same rate.  In other words, the ratio of reactants and products remains the same.
An equilibrium occurs because most reactions do not go to completion. (A reaction can only go to completion if the product is continually being removed from the system.)
A reaction in which the reactants are highly favored over products is the proton transfer of acetic acid with water: CH3COOH + H2O ⇌ H3O+ + CH3COO- The ratio of reactants to products is about 10000 to 1.
Le Chatelier's Principle (le-shat-lee-ay's principle) states that for any system in equilibrium, if a stress is applied, the system will shift to minimize the stress, where stress is defined as any change in conditions.
- Ebbing, Darrell D. and Steven D. Gammon. General Chemistry: Eighth Edition. New York: Houghton Mifflin Company, 2005.