Catholicism, also known as the Catholic Church or Catholic Christianity, is the continuation of the organized Christian church as it developed in Western Europe from the beginning to the present day. Its leader is the Pope, the bishop of Rome. (Roman Catholicism is often used interchangeably with Catholicism but there are other, smaller forms of Catholicism, so they are not exactly the same thing.)
Many other Christian traditions have existed besides Catholicism, of course. Some early forms were small and swiftly condemned as heresies, while others were large and powerful enough to branch off and continue to exist today. These include the Orthodox Church, which split from Catholicism in 1053, and Protestantism, which separated in the 1500s.
Although the Catholic Church continues to teach that it alone has carried on the true tradition of the apostolic church, the Second Vatican Council declared all baptized Christians to be "in a certain, although imperfect, communion with the Catholic Church."  So to be a Catholic today means to be a certain kind of Christian: one with beliefs, practices and traditions that differ from those of Protestantism, Greek Orthodox, Anglicanism, and other branches of modern Christianity.
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.  However, if Protestants are considered as one group, Catholics remain a minority among America's Christians.
Distinctive 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 purification before entering heaven, and the doctrine of transubstantiation (that the bread used in the Eucharist becomes the true body of Christ when blessed by a priest).
Generally, Catholic worship 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 (which they call Mass) more often than do Protestants, usually weekly.
Catholics observe seven sacraments: religious rituals believed to be commanded by God and effective in conferring grace on the believer. Other distinctive Catholic practices include veneration of saints, use of the crucifix, and the use of rosary beads in prayer.
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. 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.