A Protestant is one who is part of that branch of the Christian tree that split from the Roman Catholic Church during the early 16th century over matters of faith and practice. Matters of faith included whether the Bible was the sole authority for Christians, whether priests mediated between God and believers, and whether salvation was a gift or earnable reward. Matters of practice included whether the Church could offer/sell forgiveness of sins, whether there were two or more sacraments, and what the role of the pope was in church life.
Protestants generally believe that scripture is the sole authority in Christian life, they call salvation a free gift already given in Christ, they argue for the priesthood of all believers, and they celebrate two sacraments (Baptism and the Eucharist). Catholics generally believe that both scripture and tradition, as interpreted by the Church's teaching office, guide Christian life, that the pope is the earthly head of the Church, that priests mediate between God and believers, and that there are seven sacraments (including baptism and the Eucharist).
The name "Protestant" itself was used for the first time in 1529 to describe a group of leaders who refused to accept Catholicism within their jurisdictions. There exist today statements of agreement between Protestant churches and the Roman Catholic Church on some matters of faith and practice like justification by faith and baptism. Disagreements, however, continue to exist over things like the sharing of Eucharist, the relation between scripture and tradition, the role of teaching offices, and the role of the papacy.