A catalyst is a chemical substance that lowers the activation energy to speed up the reaction itself but DOES NOT alter the Gibbs free energy or the reactants itself. Catalysts are not used up during the process and must remain by the time the reaction went to completion.

A catalyst is a substance added to a reaction that changes the mechanism of the reaction to one requiring a lower activation energy.  As a result of the lower activation energy, the reaction moves at a faster rate.

http://www.enotes.com/catalyst-reference/catalyst