For the first thousand years of Christianity there was no "Roman Catholicism" as it is known it today, simply because there was no Eastern Orthodoxy or Protestantism to distinguish it. There was only the "one, holy, catholic church" (note: "catholic" means "universal"), affirmed by the early creeds, which was the body of Christian believers all over the western world, united by common traditions, Christian beliefs, church structure and worship.
Thus, throughout the Middle Ages, if you were a Christian, you belonged to the Catholic Church. Any Christianity other than the Catholic Church was a heresy, not one of several Christian denominations. Today, however, Roman Catholicism is not the only accepted Christian church, although the Church teaches that it is the most authentic form of the faith today.
Thus to be a Roman Catholic means to be a certain kind of Christian: one with unique beliefs, practices and traditions that are distinct from those of other Christians. Nevertheless, the Catholic Church continues to maintain that it alone has carried on the true tradition of the apostolic church and has traditionally regarded dissenting groups as heresies, not alternatives (Martin Luther was swiftly excommunicated). However, the recent Second Vatican Council declared all baptized Christians to be "in a certain, although imperfect, communion with the Catholic Church." 
The Makeup of Catholicism
Roman Catholicism did not begin at a specific point in history, like the Protestant denominations. In its long history, the church has evolved into a distinctive branch of Christianity with beliefs, practices and organization that differ from both Protestantism and Orthodoxy. (Comparison Chart: Catholicism and Protestantism)
Roman Catholicism is by far the largest Christian group. With more than one billion adherents, Catholics constitute about half of the world's Christians.  Catholicism is the majority religion of Italy, Spain, and nearly all Latin American countries. In 2001, about 24 percent of Americans identified themselves as Catholic, making Catholicism the largest Christian denomination in America (if the Protestant denominations are counted individually). The next largest denomination, Baptists, was claimed by 16 percent of Americans.  Yet if Protestants are considered as one group, Catholics remain a minority among America's Christians. (See Religion Statistics)
Roman Catholic beliefs do not differ drastically from those of the other major branches of Christianity - Greek Orthodoxy and Protestantism. All three main branches hold to the doctrine of the Trinity, the divinity of Jesus Christ, the inspiration of the Bible, and so on. But on more minor doctrinal points, there are clear Catholic distinctives in belief.
Distinctive Roman Catholic beliefs include the special authority of the pope, the ability of saints to intercede on behalf of believers, the concept of purgatory as a place of afterlife purification before entering heaven, and the doctrine of transubstantiation - that is, that the bread used in the Eucharist becomes the true body of Christ when blessed by a priest.
Distinctive Catholic PracticesCatholic Mass
With the possible exception of some Anglican churches, the Catholic liturgy tends to be more formal and ritualized than its Protestant counterparts. Services follow a prescribed liturgy and priests wear more elaborate vestments than most Protestant ministers. Catholics usually celebrate the Eucharist more often than do Protestants, usually weekly. In Catholicism, the Eucharist is called the Mass.Catholic Sacraments
Catholics observe seven sacraments, which are religious rituals believed to be commanded by God and effective in conferring grace on the believer. There are several Catholic monastic orders, the most well known being the Jesuits, Dominicans, Fransciscans, and Augustinians. Catholic monks and nuns take vows of poverty, chastity and obedience and devote themselves to a simple life focused on worshipping God.
Unlike their counterparts in both Protestant and Orthodox churches, Catholic priests take vows of celibacy. This practice is rooted in the papacy's early connections with monasticism, but has become controversial in recent years in part as a result of child abuse scandals.
Other distinctive Catholic practices include veneration of saints, use of the crucifix, and the use of rosary beads in prayer.
History of Catholicism
Roman Catholicism traces its history to the apostles, especially the Apostle Peter. St. Peter is considered the first pope, and every pope since him is regarded as his spiritual successor. This gives the leader of the church spiritual authority and provides a means for resolving disputes that could divide the church. Through trials like persecution, heresy, and the Reformation, the notion that the church leadership represents the continuation of an unbroken line from the apostles and their teachings ("apostolic succession") has contributed to the survival of Christianity.
The Catholic Pope
However, the idea of the "pope" did not exist from the beginning of the church. It was not until several centuries after Christ that the church began to develop into the "Roman Catholic Church" as we think of it today, with its particular doctrines, practices, and hierarchical system of authority. Thus Catholics and non-Catholics alike are able to claim they are most faithful to the message of the apostles and the early church. From the Catholic perspective, the early church is faithfully continued in the developments of later centuries, while non-Catholics tend to regard the church as having corrupted the original message of Christianity.
In the years of persecution prior to the Emperor's conversion, the church was focused primarily on survival. There were prominent church leaders whose authority was recognized - primarily those who had known the apostles - but no central authority.
But with the conversion of Emperor Constantine in 318 AD, the church began to adopt a governmental structure mirroring that of the Empire, in which geographical provinces were ruled by bishops based in the major city of the area. Soon, the bishops of major cities in the empire emerged as preeminent, including the bishops of Jerusalem, Alexandria, Antioch, Rome, and Constantinople. It was natural that Rome would eventually become the most important of these. It was not only the capital of the empire, but the city in which the apostles Peter and Paul were believed to have been martyred.
The Roman bishop Leo I (440-461) is considered the first pope by historians, as he was the first to claim ultimate authority over all of Christendom. In his writings one can find all the traditional arguments for papal authority, most notably that which asserts Christ had designated Peter and his successors the "rock" on which the church would be built.
Leo's claims were strengthened greatly by his own impressive career as Bishop of Rome. In 445 he earned the express support of Emperor Valentian, who said the Bishop of Rome was the law for all. In 451, he called the important Council of Chalcedon, which put to rest Christological issues that had been plaguing the church. In 452, he impressively saved Rome from Attila the Hun. It is said that the Pope met the warrior at the gates and somehow persuaded him to spare the city. Legend has it that Attila saw Peter and Paul marching along with Leo to defend their city. In 455 he was not as successful with Vandal invaders, but led negotiations with them and succeeded in preventing the burning of Rome (it was, however, plundered).References
- Papal encyclical Unitatis redintegratio, 3.
- John Bowker (ed.), Oxford Concise Dictionary of World Religions, 490.
- Catechism of the Catholic Church Online text of the Second Edition English translation, with corrections made by the Pope in 1997.
- Catholic Answers Library Articles related to Catholic apologetics and evangelism.
- Catholic Encyclopedia Searchable online text of the Catholic Encyclopedia. An excellent resource with extremely detailed articles on just about every Christian topic. Note that it was originally published circa 1915, so it lacks information on Vatican II or other modern developments.
- Official Site of The Vatican A beautiful and useful site with resources about Vatican City, the pope, liturgical celebrations, and other Catholic topics. Available in several languages.