You could say that catalysts and inhibitors are opposites of each other. The both affect how chemical reactions proceed.

A catalyst increases the rate (the speed) of a reaction. This effect can be so large that it may seem that the catalyst makes the reaction possible. Imagine a reaction that would take years to finish without a catalyst but that is sped up to take just minutes by a catalyst. A catalyst, however, can only accelerate reactions that are energetically "allowed" to happen, it cannot make reactions happen. Catalysts are very important in many areas. In biochemistry, enzymes are catalysts of biochemical reactions in the body. In chemical industry, catalysts allow important reactions to be performed quicker or at lower temperatures or under otherwise milder conditions. In pharmaceutical research, most reactions to make new medicines will make use of some kind of catalyst.

An inhibitor, on the other hand, slows down or even stops chemical reactions. This can be desirable or unwanted. For example, a scientist wanting to isolate a protein from a biological sample may add inhibitors for enzymes that break down proteins. In this case, the inhibitor's effect is desired. In another example, a scientist may have to remove all air from their setup (and replace it with another gas), because the oxygen in the air can inhibit certain chemical reactions.