A Cathedral is a very large and very fancy church. The Catholic Church had a lot of money and saw building large churches as a way to give honor to their god. Remember, religion was the controlling force in everyday lives. Many of these Cathedrals are still around to visit today.
Cathedrals were usually built in the shape of a cross with very high ceilings. But think how hard it must be to light this type building with just candles. As additional honor to their god, art work and stained glass windows were incorporated into most cathedrals.