The full complete answer can fill a whole book. But the nutshell
answer is this: The difference between Catholicism and
Protestantism (Not Christianity which is a local Singaporean
misconception as Catholics are Christian too) is that Catholics
have chosen to remain faithful to the original teachings that have
been passed down for close to 2000 years now, while Non-Catholics
on the other hand have abandoned 2000 years of Christian theology
and re-invented their own version of Christianity based upon their
reading and understanding of the Bible.
If you want more information look up an encyclopedia or a good
history book on the protestant reformation.