Nothing. Catholicism IS a Christian religion. There are variations within Christian denominations, but Catholics are Christians just like Protestants are.
Protestants are Christians who deviated from Catholicism, and the differences are largely ritualistic. Though some other basics are the differences between the denominations' views on the importance of the Scripture, the importance of the Pope, the exact nature of how one becomes saved and salvation, the inclusion of Saints, and what happens after death.
But still, at the end of the day, they're all the same religion: Christian.