A catalyst is used to speed up a chemical reaction. catalysts work by lowering the activation energy of the reaction which allows it to proceed at a faster rate. catalysts are also not affected by the over all chemical reaction (they appear in both the reactant side and product side unchanged but can be changed somewhere in the reaction mechanism before being changed back).
I hope this helps. Let me know if anything is unclear or if you just want me to explain anything in more detail.
