A catalyst speeds up a chemical reaction by lowering the activation energy for the reaction. Catalysts are specific to which chemical reactions they work for.
Many catalysts work by providing a surface for a molecule to form temporary bonds with. This temporary coupling is weaker than the original bonds of the molecule and allows the molecule to interact with the next part of the chemical reaction with lower activation energy. Catalysts must have a reactive surface layer in order to cause the temporary bonding, while not being so reactive that they become part of the resultant product.
Typically the catalyst is left unchanged at the end of the chemical reaction and is only a helper in getting the chemical reaction to occur. The actual methods of catalyst function are many, and there are even examples of catalysts changing during the course of a chemical reaction and then returning to their original form. Catalysts may also increase the number of steps in a chemical reaction, while simultaneously lowering the energy required, thereby making it easier to reach the final desired product.
Biological catalysts are called enzymes. Enzymes have specific locations called “active sites” that take in substrates, weaken bonds, cause chemical reactions, and create new products.