Christianity as a world religion has been around for about 2,000 years. It has grown and changed over the centuries, but Christian churches with any kind of orthodox beliefs all affirm Jesus Christ as the incarnation of God. They believe He came to die and secure salvation for all who believe in Him. This is the basis of all Christian theology.
In the intervening centuries, however, Christian beliefs and churches have grown and changed. In the early days of the church, people met mostly in each other's homes. Communal living, in fact, was encouraged. As beliefs were further codified and the books of the New Testament finalized, the Catholic Church began to take shape. By 1100 A.D., Christendom was a major political and social force throughout the known world. Popes of the Church held great influence over world rulers, since they had the ability to put entire nations under interdict — that is, no one could receive the holy sacraments, which was tantamount to excommunication. It was the threat of interdict and excommunication that forced England's King Henry II to make a pilgrimage to the tomb of St. Thomas A'Becket after having ordered his murder.
Reform was bound to happen in such a large organization as the Catholic Church, and in 1517, it started when Martin Luther, a German priest, composed and published his "95 Theses." This was an indictment against the sale of indulgences. People were paying money to touch or otherwise venerate objects in the hope of buying their salvation. Luther abhorred this practice and preached against it. When he was excommunicated in 1521, those who followed his teachings were known as Lutherans and, later, as Protestants. By 1547, England's King Edward VI was the first Protestant ruler in Europe.
There are a significant number of differences among mainline Christian denominations, but they break down roughly along the lines of certain religious scholars. One of the first great Protestant scholars after Luther was John Calvin. He believed in the doctrine of predestination, that is, that humans were born to accept Christ, or born to be damned, and nothing they could do changed this. Although this particular belief has fallen out of popularity over the years, the Presbyterian and Baptist churches are the prominent Calvinist denominations.
When John Wesley began preaching in 1725, he was an ordained member of the Anglican Church, and remained so until the end of his life. His belief in salvation by faith alone, his repudiation of predestination, and his belief in entire sanctification started the Methodist denomination and its offshoots.
The Anglican and Episcopal churches themselves are the closest Protestant equivalents to the Catholic Church, although the Methodists do share some similarities, too. They differ from the Catholics mainly in that they do not recognize the authority of the sitting Pope as any other than a priest — no holier or better than others. There are other theological differences, as well, such as the ability of priests to marry and less emphasis on the veneration of saints.
The 1906-1909 Azusa Street Revival in California ushered in the charismatic movement. These churches, such as Assemblies of God, put great emphasis on the gifts of the Holy Spirit, especially speaking in tongues. They also focus on divine healing, prophecy, and other outward signs of the presence of the Holy Spirit. In basic theology, they are more Wesleyan, since they believe in salvation by faith alone.
Queen Elizabeth I of England faced great religious strife during her reign. Her take on the situation was that "There is but one Jesus Christ. The rest is a dispute over trifles." She was correct in that some of the main differences among mainline Christian denominations are over practice, rather than theology. Is one immersed completely at baptism, or will being "sprinkled" suffice? Arguments have spring up over this question.
Individual church traditions also dictate different worship styles. In fact, anyone who wishes to join a Christian congregation can probably find one with a worship style that suits him exactly. The message is often the same, but the manner of conducting the service differs.
This then, is at the heart of all differences among denominations: all Christians, regardless of denominations, confess Jesus Christ as Lord and worship Him. Regardless of how the worship styles and other theological beliefs may differ, this is what all members confess and believe.