http://en.wikipedia.org/wiki/ProtestantsThat article encompasses just as much as I would tell you.
Basically, Catholics are of the original Church of Peter, Protestants are not. Protestants broke away from the Church during the Reformation, a period of new ideas concerning one's reconciliation with God through Christ. The reformers accused the Church of being Corrupt (accusations which were, in at least some major positions, true) and the Church accused the reformers as heretics because of their ideas.
The core interpretational differences could be found in the 5 solas of protestantism, namely:
"* Solus Christus: Christ alone.
The Protestants characterize the dogma concerning the Pope as Christ's representative head of the Church on earth, the concept of meritorious works, and the Catholic idea of a treasury of the merits of saints, as a denial that Christ is the only mediator between God and man.
* Sola scriptura: Scripture alone.
Protestants believe that the teachings of the Roman Catholic Church obscure the teachings of the Bible by convoluting it with church history and doctrine.
* Sola fide: Faith alone.
Protestants believe that faith in Christ alone is enough for eternal salvation (as stated in Ephesians 2:8-9), whereas Catholics believe that the phrase "faith without works is dead" (as stated in James 2:20) points to salvation needing to be earned. Protestants, pointing to the same bit of scripture, believe that practicing good works attests to one's faith in Christ and his teachings.
* Sola gratia: Grace alone.
The Roman Catholic view of the means of salvation was believed by the Protestants to be a mixture of reliance upon the grace of God, and confidence in the merits of one's own works, performed in love. The Reformers posited that salvation is entirely comprehended in God's gifts, (i.e. God's act of free grace) dispensed by the Holy Spirit according to the redemptive work of Jesus Christ alone. Consequently, they argued that a sinner is not accepted by God on account of the change wrought in the believer by God's grace, and that the believer is accepted without any regard for the merit of his works — for no one deserves salvation.
* Soli Deo gloria: Glory to God alone
All glory is due to God alone, since salvation is accomplished solely through his will and action—not only the gift of the all-sufficient atonement of Jesus on the cross but also the gift of faith in that atonement, created in the heart of the believer by the Holy Spirit. The reformers believed that human beings—even saints canonized by the Roman Catholic Church, the popes, and the ecclesiastical hierarchy—are not worthy of the glory that was accorded them."