Catalysts are substances added to reactions that change the rate of the reaction. They can either increase the rate of the reaction in the case of positive catalysts or slow down the rate of the reaction in the case of inhibitors.
The characteristic feature of catalysts is that though they affect the reaction in which they are used they are not consumed in the reaction. A catalyst may bind with the reactants during a stage of the reaction but they unbind as the reaction proceeds and are found in their original form and state at the end of the reaction. This makes it possible to use the same catalyst to catalyze a reaction repeatedly.