A catalyst increases the rate of a reaction without being used up. At the end of the reaction it is chemically unchanged, so it can be taken out and used again. Only a small amount is needed.
A catalyst provides an alternative pathway for the reaction. It does not change the products and it does not change the amount of product. If a reaction makes 50 cm3 of gas without a catalyst, it still makes 50 cm3 with one, but it gets there sooner.
Catalysts are often specific to one reaction, so a catalyst that works for one reaction may not work for another. For example, manganese(IV) oxide, MnO2, is a catalyst for the decomposition of hydrogen peroxide into water and oxygen: 2H2O2 → 2H2O + O2. The black powder is left unchanged at the end, so its mass is the same as at the start.