If a change (stress) is imposed on a system in equilibrium, the position of the equilibrium will shift in a direction to partially offset the change (stress).

The changes that can act to shift equilibrium are changes in concentration, pressure for gaseous reactants or products, and temperature.

Increasing the concentration of a reactant shifts toward product. Increasing the temperature of an exothermic reaction shifts to reactants. If the number of molecules of products in the balanced equation (not necesarily in an actiual system) is smaller than the number of molecules of reactants, then reducing the volume shifts to products.

While a catalyst will allow a system to attain equilibrium more quickly, it does not affect the equilibrium position.

If a system is at equilibrium and either a reactant or product is added then the reaction will shift so as to reestablish the equilibrium by partially consuming the added substance. In the diagram below the system at equilibrium and the newly established equilibrium represent two different positions. Between the two positions the equilibrium is in flux (shifting) which is expected when Q is not equal to K.

LeChatelier's Principle [local]