Some reactions are over in an instant and others take years. The rate can be changed, and a catalyst is one way to change it.
A catalyst speeds up a chemical reaction without being used up in the reaction.
It does not change the amount of product that can ultimately be formed. It only changes how quickly the reaction happens, and it does not start a reaction that would not happen anyway.
Different reactions need different catalysts, and only a small amount is needed. In living things the catalysts are enzymes, which allow reactions that would otherwise be far too slow to happen at body temperature.
Rate can be changed in other ways too: raising the temperature, increasing the concentration of a solution, and breaking a solid into smaller pieces to increase its surface area all make a reaction faster.
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