A catalyst is a substance that is used to help start or cause a chemical reaction to progress. Sometimes mixing chemical reagents together will produce no chemical reaction on its own but the introduction of a catalyst will help cause the chemical reaction to occur. Please note that the catalyst itself is unchanged as a result of the reaction. One example would be combustion. Propane gas will combust and burn in the presence of oxygen gas but only when a sudden burst of energy (usually a spark) is introduced as a catalyst. If by a normal scenario you mean a real life industrial example, metal catalysts are used in the petroleum industry to help break down larger hydrocarbon chains found in crude oil into smaller, more useful hydrocarbon chains (gasoline, diesel, jet fuel) more efficiently. The smaller fuel molecules are then separated and collected by distillation.