The Catholic Church is not only the deliverer of the Gospel and "a pilgrim now on earth...necessary for salvation" (CCC 846) it is also a hierarchical institution which has existed for over two thousand years, a vast period of time almost unmatched in history.
The head of the Catholic Church is its founder Jesus Christ. As his vicar on Earth Jesus appointed Peter, the first Bishop of Rome. Every Pope since has been the successor of Peter. For most of the last 2000 years the Pope has resided in Rome. As the Supreme Pontiff the Pope presides not only over the Latin Church, that is the Roman Catholic Church (Latin Rite), but also the 22 Eastern Churches in communion with Rome. The Pope is also the head of state of the Vatican, and as such is entitled to the same protocol as any head of state. As an independent nation the Vatican accepts and appoints ambassadors to other nations. Papal ambassadors are called nuncios and work for the Vatican's Secretariat of State.
The pope is elected by the College of Cardinals. Cardinals are members of the clergy who have been appointed by the pope. Most Cardinals today are bishops, but the Pope can appoint any clergyman, deacon, priest or bishop, a Cardinal. Any member of the clergy is eligible to be elected Pope, but in practice elections usually select a member of the College of Cardinals to become Pope.
The Pope is assisted in carrying on the administration of the Church by the Roman Curia. The curia is divided into the Secretariat of State, the Roman Congregations, the Tribunals, the Pontifical Councils, The Pontifical Commissions, the Swiss Guard and the Pontifical Academies.
Some of the more important and well known Congregations are the Congregation for the Doctrine of the Faith, The Congregation for Divine Worship and Discipline of the Sacraments and the Congregation for the Causes of Saints. The Congregation of the Doctrine of the Faith, sometimes called the Holy Office, is responsible for safeguarding the doctrine of the faith. They do this by promulgating documents which support Catholic doctrinal positions. They also review the publications of Catholic theologians and others for conformity to Catholic Doctrine. The Congregation can require that both clergy and lay theologians recant positions that conflict with Catholic doctrine and even punish those who refuse. They can also correct any Catholic who is guilty of heresy or other action that would scandalize the faith.
The Congregation for the Causes of Saints oversees the process which leads to the canonization of saints. After the case is prepared it is presented to the pope who decides whether to proceed with beatification or canonization.
The Congregation for Divine Worship and Discipline of the Sacraments handles affairs relating to liturgical practice of the Latin Rite. The Eastern Churches handle their own liturgical matters. This includes the liturgical calendars and music as well as the propers of the Divine Office and the Mass. This is the office responsible for ensuring that liturgical abuses are avoided, usually by informing and supporting bishops, who are responsible for the proper celebration of the liturgy in their diocese. The proper celebration of the Extraordinary Form of the Roman Rite, that is the Mass and Sacraments performed in Latin according to the forms used prior to Vatican II is vested in the Pontifical Commission Ecclesia Dei.
The Church community is governed according to formal regulation set out in the Code of Canon Law. The Catholic Church has a complete legal system, one of the oldest in the world. This includes a complete appellate courts system. Most of the cases heard under canon law today are marriage annulment cases, but cases also include petitions for redress of punitive and non-punitive actions such as excommunications.
Worldwide the Church is divided into 2782 episcopal sees, which are called dioceses. Each diocese is headed by a bishop. Some few are lead by patriarchs. Eastern Church sees are called eparchies and lead by an eparch.
Vatican II established that bishops responsible for specific geographic areas be organized into national episcopal conferences. The United States Conference of Catholic Bishops is the national episcopal conference for the U.S. Many tasks and authorities are relegated to the episcopal conferences, particularly in the area of setting liturgical norms for the Mass. The USCCB also works with the International Commission on English in the Liturgy (ICEL) and the Bishop conferences of other English speaking countries on approved translations of the Liturgy. All liturgical documents are composed originally in Latin.
Besides the bishops, priests and deacons which are in the dioceses there are members of the church, both clergy and the laity, who are in the consecrated life. The majority of these people are in one of the religious orders. Members of religious orders are usually under the authority of a superior general. Usually they are independent of the authority of the local bishop, though often they are required to coordinate with him in matters that effect the faithful in his diocese. Some few religious orders, such as the Society of Jesus (the Jesuits) report directly to the Pope. Some orders. Like the Benedictines have houses which are independent.
Opus Dei is a unique order called a Prelature. It is a personal prelature which means that it's bishop's jurisdiction covers all persons in Opus Die, laity and clergy where ever they are. Such people are not under the authority of the local bishop although they are expected to coordinate with the ordinary within his diocese.
Diocese are typically organized into ecclesiastical provinces which are headed by an archbishop. Such archbishops are called metropolitan bishops, and exercise a limited degree of authority over his suffragan bishops. Often dioceses are divided into vicarates, each having a diocesan vicar.
Each parish is by design a geographical unit with specific boundaries. A pastor administers the parish and is sometimes helped by a parochial vicar and other parish priests. Often a group of parishes are assigned to a single pastor who is responsible for their administration. Deacons, although they work in parishes are assigned under the authority of the bishop and although they help the pastor they are assigned by the bishop and actually work for him.
The pastor is typically advised by a number of parish committees, including the parish council and finance committee.
Besides the hierarchical structure of the church described above there is a separate structure which is made up of the various religious orders, many of which contain both clergy and members of the laity who have given themselves up to a consecrated life.
Initially the orders were founded on the three principles of chastity, poverty and obedience. These are called solemn vows. Some members of religious life take simple vows. The basic difference is that in simple vows a person maintains the right to own goods while in solemn vows the person renounces the right of ownership. Most religious orders, even those under solemn vows allows that the order may own goods, but there are exceptions to even that, with a few order completely dependent on the will of God for their maintenance.
Though in most orders (and in single consecrated life) a vow of chastity is required there are exceptions. Members of the Secular Franciscan Order (SFO) who are members of the Third Order of Franciscans may be married and are not bound by public vows. There are other orders where married members make public profession and are bound to marital chastity, that is bound to their spouse.
Some orders are have houses that are autonomous, as are the Benedictines. Others are more or less hierarchical in organization having a single leader, usually appointed by the Pope, though sometimes the members of the order are allowed to nominate an individual for the Pope's concurrence.
Many members of the religious orders serve others, the poor, the sick and the disenfranchised. Others are teachers, councilors, pastoral ministers, religious educators and even parish priests.
The structure of the Church is forever changing, though usually slowly and in small ways. New orders are founded and new commissions established, and so too do orders die out and curial offices become vacant or combined. This is a short snapshot of the Church structure as it exists today.