Protestantism
Protestantism is an umbrella term for multiple traditions of Christianity which have a common lineage to the Protestant Reformation. While a variety of definitions exist of the theological boundaries of Protestantism the three basic branches of Protestantism are Lutheranism, Reformed Christianity, and Anglicanism. Other groups which can be considered Protestant are Methodists, Moravians, Congregationalists, and Baptists.
Theology
The basic unifying principles of Protestant theology are the unique Supremacy of Scripture in matters of doctrinal dispute, the doctrine of Justification by Faith Alone, and the rejection of many of the theological doctrines of Roman Catholicism such as their view of the Papacy, many of the Marian dogmas, Transubstantiation, Indulgences, and Purgatory. Other doctrines such as Penal Substitutionary Atonement, the Infallibility of Scripture, Total Depravity, etc. are sometimes also considered defining features of Protestant Theology.
66 Book Canon
Protestants hold to a 66 Book canon of the Bible. While the Deuterocanon or Apocrypha were often included in Protestant Bible translations such as the King James Version, and the Deuterocanonical books are held to be edifying reads, they are not considered sure grounds for the establishment of doctrines, due to the disputed nature of their inspiration.