…increase the rate of a reaction while remaining unchanged chemically themselves.
There are two distinct processes…
1. The reactants are ADSORBED on to the catalyst surface, making it easier for the reactants to combine. The reactants like the surface of the catalyst, so they tend to move towards it, making collisions and hence reactions more probable.
2. The catalyst undergoes an intermediate, short-lived reaction, returning to its original state after the reactants have reacted together. Biological catalysts, called ENZYMES work like this.
For more details, download this Word document explaining how catalysts work