To understand Catalysts, it is necessary to understand what Catalysis is.
Catalysis is the change in rate of a chemical reaction due to the participation of a substance called a catalyst.
In other words, a chemical reaction that might occur naturally is sped up or slowed down by the introduction of a new substance which is not directly involved with the reaction itself. Catalysts are not used up by the reaction, but they might be damaged or made useless for further participation in the reaction process.
The two main forms of Catalysis are Heterogeneous and Homogeneous.
A Heterogeneous catalyst operates separately from the reactants; it is not in the same Phase, or uniform state, as the chemicals it is affecting. These are important for chemical production on a mass scale; many chemicals that do not naturally occur in large quantities can be synthesized quickly this way. An example of Heterogeneous Catalysis is the Haber Process, which synthesizes Ammonia from Nitrogen and Hydrogen using Iron filings.
A Homogeneous catalyst operates with the reactants; it is in the same Phase, or Uniform state, as the chemicals it is affecting. These catalysts will be mixed with or placed on the reactants directly and then remain in the solution while the reaction occurs. An example of Homogeneous Catalysis is destruction of Ozone by CFCs, which bond with free Oxygen atoms, preventing the formation of new Ozone.