Catalyst: A substance that changes the rate of reaction but emerges chemically unchanged at the end of the reaction

Catalysts can speed up or slow down the rate of reaction
The catalyst does not change the overall energy change. However, since less energy is needed to start the reaction, more reactant molecules have enough energy and so more collisions are successful.

Examples of catalysts in everyday life:

Catalytic converters in car exhausts use platinum and palladium. They lower the activation energy needed to convert harmful gases to harmless gases
Enzymes are large complex protein molecules that act as catalysts in a biological reaction. Alcoholic drinks are produced using an enzyme called yeast

Each enzyme molecule has a part with a specific shape called the active site
The reactant molecules (substrates) fit the shape of the active sit, a bit like a lock and key
Therefore each enzyme catalyses only one reaction and we say that enzymes are specific
Enzymes are sensitive to changes in temperature and pH as these can denature their molecules (make them change shape)