Christianity is not a monolithic religion, nor has it been for several centuries. The earliest Christian communities had various tensions toward retaining Jewish practices and creating a distinct identity as believers. Amongst some Christians, Gnosticism was accepted, among others, it was anathematized. As these differences were sorted out during the ecumenical councils over a period of centuries, Christians still continued to split into various bodies with distinct national heritages and ecclesiastical authority.
Broadly speaking, the largest split amongst Christians has been between Eastern and Western Christianity. Easterners emphasized personal experience of God, tradition, monasticism, and the primacy of the Greek language and the Greek translation of the Bible. The Western church was marked by rational explanations of faith, the primacy of the Bishop of Rome, and a Latin-based liturgy.
In addition to historical schisms between Christian bodies, there are hundreds of millions of independent Christians today, including many in the United States and sub-Saharan Africa. The latter have been particularly influenced by the rise of Pentecostalism in the 20th century. A booming house church movement has also taken hold in mainland China. A much smaller movement of Christians seeking the Jewish origins of Christianity has emerged in the past 150 years as well.
Overview of denominational families 
Eastern Christianity 
The Eastern Orthodox Church is made up of approximately 17 independent ("autocephalous") churches - the numbering can differ depending on one's perspective on the Orthodox Church in America. These churches have jurisdiction over national boundaries and share a common communion while retaining their individual liturgies and customs. All recognize the special place of the Ecumenical Patriarch of Constantinople as primus inter pares or "first among equals" as a kind of guiding figure amongst the other Patriarchs. Several smaller churches are autonomous and under the authority of autocephalous bodies.
Eastern Orthodoxy and Roman Catholicism represented a majority of Christendom and remained communion with one another until the East-West or Great Schism of 1054, in which the Ecumenical Patriarch and Pope excommunicated one another. It has only been since the efforts of Pope Paul VI and the Second Vatican Council in the 1960's to renew fellowship that the first steps toward reconciliation have been made.
Oriental Orthodox 
The Oriental Orthodox Church is more loosely affiliated culturally than the Eastern Orthodox (different churches have even maintained different Biblical canons than one another), but the role of the Coptic Pope is stronger ecclesiastically than the Ecumenical Patriarch. For instance, when Eritrea gained independence from Ethiopia in 1993, Pope Shenouda III made two distinct churches for the two communities. The schism with the Oriental churches came about as a product of the Council of Chalcedon in 451 and the resulting Chalcedonian Creed which established a particular formula for understanding how Jesus can be both divine and human in full measure. The Christology of the Orientals has been labeled monophysite, but the Church defines itself as miaphysite - the differences go back to the original Christological controversy of the fifth century, with the former denying that Jesus had any substantial human nature. Today, Orientals recognize that these differences were largely political and there is no insurmountable theological differences between them and Eastern Orthodox or Catholics. Similar to the Easterners, Orientals use icons in worship, have a strong monastic tradition, and have focused on the lives of saints for spiritual guidance.
The Assyrian Church of the East became a separate body after the Council of Ephesus in 431, a gathering of Christians to combat the heretical group lead by Nestorius, leading to charges that the Assyrians were themselves Nestorians. Like the Oriental Church, the Assyrians deny this and have made several important declarations in the 20th century to affirm their common ground with other Christian bodies. In the mid-sixteenth century, several Assyrian churches broke communion with the Patriarch of Babylon, spiritual head of the Church of the East and joined into communion with the Catholic Church becoming the Eastern Catholic Chaldean Rite. Due to political instability in Iraq and Assyrian immigration, the church is headquartered in Chicago, the United States.
Eastern Catholicism 
While the Latin Rite makes up over 98% of the believers in the Catholic Church, there are 15 Eastern Rites which are allowed to conduct their own liturgy and maintain many of their own distinct doctrines and practices while acknowledging the primacy of the Bishop of Rome, as an authoritative head to their church as well.
Western Christianity 
Catholicism is the largest denominational family amongst Christians, and the Roman Catholic Church is the largest institution of any kind on the face of the Earth, largely due to missiological efforts in Latin America and Africa. In addition to believers who are the product of Portuguese and Spanish colonialism, Catholicism has a strong history in Western Europe, where it remained virtually the sole church in the West from the time of the Schism through the Anglican, Magisterial, and Radical Reformations of the 15th and 16th centuries.
The Catholic Church is united by several distinct traditions, possibly the most important of which is the Pope - Bishop of Rome and the monarch of Vatican City. The Pope's ecclesiastical see is known as the Holy See and is given a priority in terms of both honor and authority amongst all other Catholic bishops. Catholics have rich traditions of doctrine, canon law, sainthood, and sacred architecture.
Protestantism is not a single church body or set of formally-related organizations, but a grouping of various church families whose history extends to the Reformations in 16th century Europe. The main families are the Lutheran, the Reformed and Presbyterians, and the Anabaptists. Protestantism has spread as widely across the globe as has Catholicism, and has developed into a wide variety of national expressions as well as being foundational in international ecumenical movements such as the World Council of Churches.
Certain pre-Reformation groups are frequently included in discussions about Protestantism, such as Waldensians and Moravians, who are legacies of reformation movements lead by Peter Waldo in 12th-century Italy and Jan Hus in 15th-century Bohemia. Some classification systems also include Anglicans as well, as the Church of England and the Roman Catholic Church ceased to be in communion roughly concurrent with the Reformation. Since the Reformation, large Protestant groups include Baptists, Methodists, the Disciples of Christ, the Evangelical Christian Church in Canada, and Adventist churches that come from the Restorationist movement of mid-18th century America. Some of most particular Christian groups come from the latter movement, including Mormons and the Jehovah's Witnesses. Depending on the definition of what constitutes Christianity, those movements may not even be considered Christian due to their unique understandings of virtually every Christian doctrine.
Anglicanism is a form of Christianity with commonalities with both Protestantism and Catholicism. The Anglican Church was formed by Henry VIII at roughly the same time as the self-definition of Protestantism, and there are some commonalities between Protestantism and Anglicanism. The Church of England ceased to be in communion with Rome as a result of Henry's actions and his insistence that the King of England be head of the British church. At the same time, the emphasis on ritual and liturgy in the Roman Catholic Church is also found, to some extent, in the Anglican Church. Anglicans also stress the common lineage their clergy have with Catholics - a doctrine known as apostolic succession, which ties present-day religious workers with the original twelve apostles of Jesus.
Independent churches 
Some churches that do not identify with any denomination. Charismatic Christianity and Pentecostalism are two similar movements founded in the twentieth century which prioritize an individual's experience of God through the Holy Spirit, including the reception of spiritual gifts such as glossolalia, spiritual healing, and prophecy. Charismatics have generally stayed within their original church bodies, whereas Pentecostals have created their own denominations and separate churches.
Evangelicalism is another movement that exist both with established church groups and entirely independent churches, some of which have evolved out of small house churches and private Bible study groups into larger networks of churches, particularly in America. Evangelicals are similar to fundamentalists in some of their emphasis on the priority of the Bible, but are not as hostile to modernity nor mainstream politics. They have also been associated with a social Gospel in their insistence on providing services to underprivileged populations, particularly through missions work.
Both of these movements are Western although they have typically do not have formal ties to other historically Western churches.
Another important cross-denominational movement in the 20th century has been ecumenism: a cooperation of Christian bodies that can be as simple as coordinating common efforts in social services, writing common theological statements, the restoration of communion between bodies, and even the creation of entirely new denominations themselves, such as the Uniting Churches. The World Council of Churches (WCC) is an international body whose membership includes a majority of the Christian world if one includes Catholicism (the Roman Catholic Church is not a member of the WCC, but works with it closely on several initiatives.) The National Council of Churches is an American equivalent that has similar goals as the WCC.
- Oxford Encyclopedia of Christianity - This is an excellent and broad overview of Christianity; Oxford has published similar volumes on Christian history and the Bible as well.
GOD IS A TWAT