Christianity is the belief that Jesus Christ was the son of God sent to save us from sin, he died on the cross, then was resurrected and taken to heaven by God.

Catholicism is one of many sects of the christian religion. The differences between the different sects are varied. In the broadest of strokes the differences comes down to the way they interpret the bible and the way they practice their religion. To get into more specifics than that and you go deep into the theological weeds.