A catalyst increases the rate of a reaction by providing an alternate mechanism in which the overall activation energy is lower than in the uncatalyzed mechanism. Thus a greater proportion of the molecules are likely to possess or exceed this new E_a and so the reaction proceeds more quickly at the same temperature.

In a mechanism catalysts can generally be recognized by their initial appearance as reactants and their subsequent reappearance as products. Since they can be found on opposite sides of the reaction arrows, they do not appear in the overall balanced equation.

So while it is correct to say that catalysts are not consumed or used up in a reaction, they certainly become involved and are often changed during the course of the mechanism---but they are regenerated by the end of the sequence.