The most important difference between the resulting philosophy of Luther was the reliance on the Bible as the final spiritual authority. The Catholic Church claimed (and still claims) authority through tradition--Peter the apostle was the founder of the church to whom Christ granted the keys to heaven. Spiritual truth can be changed through the Pope who is considered a direct link to God. To restate the difference, Lutherans claim the Bible as their ultimate source of spiritual truth whereas Catholics assert that the Pope (i.e., the church) is the source.
The namesake of the Lutheran Church is Martin Luther, a Roman Catholic monk who lived in Germany in the medieval era. He questioned Church authority on a few matters during his day. He, and many in agreement with him, were excommunicated by papal bull and the Lutheran Church was born. The main objections that Luther voiced involved the compulsory celibacy of priests and clergy, the sale of indulgences, works righteousness, Marian dogma exulting the Virgin to co-redeemer and mediatrix, the infallibility of the pope, and the role of the bible as a static authority on matters of belief and practice.
The Roman Catholic Church and the Lutheran Church have many similarities. Both are theologically sacramentarian, both are historic and liturgical Churches (follow the order of the mass and the liturgical calendar), and both have episcopal forms of Church governance (while the Lutheran Church employs some congregational polity.) The Lutheran reformation, along with the Anglican reformation, was a "conservative reformation" unlike the Reformed movement, which is why the Lutheran and Anglican/Episcopal Churches more closely resemble the Roman Catholic Church, in beleif and practice, than they do protestant Churches. Some Lutherans do not believe that the term "protestant" appropriately describes the Lutheran Church, due to implied associations with the Reformed and "Evangelical" Churches.
The cardinal doctrine of the Lutheran Church is that salvation is by the grace of God alone and that man can do nothing to save himself by his own works; and that good works are the fruits of salvation, not a means to attaining it. Faith is the vehicle of that grace and is awarded as a grace from God himself. Thus, the mantra of Lutheran theology that the means of salvation is: "Sola gratia. Sola fide." (Latin for "only by grace through faith.") In 1999, the Lutheran World Federation and representatives of the Roman Catholic Church officially signed a joint declaration stating, "By grace alone, in faith in Christ's saving work and not because of any merit on our part, we are accepted by God and receive the Holy Spirit, who renews our hearts while equipping us and calling us to good works." The Lutheran and Roman Catholic Church continue ecumenical dialogues, but have been unable to totally agree on some matters of faith. Despite those theological differences, the two Churches continue to work together on charitable and humanitarian efforts throughout the world.