Catholicism is a branch of Christianity, as are the Orthodox, Protestants, Evangelicals, etc. While they all disagree on some points of doctrine and ritual, their most fundamental beliefs are the same: That there is only One God, that Jesus Christ is the Son of God, that He is the Messiah, and that a human must love his neighbour as he does himself (though this last one is conveniently forgotten by Christians across the board).
It is important to stress how they are not different religions, but different sets of beliefs and practices within the same religion. Ultimately, one can be a Christian and not be a Catholic, but one cannot be a Catholic without being a Christian.
