Broadly Speaking, the differences between Protestants (of which there are many subsets) and Roman Catholics are:
The Bible:
Protestants see the Bible as the sole source of God's wisdom while Catholics are influenced by centuries of Church teaching and insight
The role of Good Works:
Protestants believe that salvation comes from faith alone while Catholics believe that faith must be accompanied by good works.
The Pope:
Catholics believe that the Pope is the successor of Saint Peter while Protestants don't see much Biblical authority for even having a Pope.
The Eucharist:
This is the big one. Con v. Transubstantiation. Catholics believe that you are eating the body and blood of Christ while Protestants believe it is a symbol.
Others:
Role of Mary, Saints, etc.
Role of the Sacraments
Priestly celibacy