Catalysts are any substance that affects chemical reaction; a catalyst can slow a reaction, or speed it up. The catalyst is not directly involved with the reaction itself, only serving to act on the speed; however, the catalyst can be rendered useless by the reaction, in which case it must be reintroduced each time the reaction occurs.
The two classes of Catalyst are Homogeneous and Heterogeneous.
A Homogeneous catalyst is a substance in the same phase, or state, as the reacting chemicals. If a gaseous reaction is occurring, a homogeneous catalyst will also be a gas. An example of this is alcohol dehydration; when changing ethanol to ethylene, sulfuric acid is used as a catalyst, but does not remain in the final substance.
A Heterogeneous catalyst is a substance not in the same phase as the reacting chemicals. If a gaseous reaction occurs, a heterogeneous catalyst can be a solid or a solid. An example of this is fluid catalytic cracking, which separates crude oil molecules; the crude oil is in a vaporous state, while the cracking catalyst is usually a powder, or solid state.
homogenoues and heterogeneus