When he established His Church, Jesus placed the Apostles in charge of caring for the faithful, of teaching them the faith and caring for their souls. And He placed Peter at the head of the Apostles. Through Apostolic Succession, that same hierarchy willed by Jesus, exists today in the Church with the Pope (the successor of St Peter) at her head, leading the Bishops (the successors of the Apostles) who themselves lead the faithful in their local Churches.
At the head of the College of Bishops and called to lead the whole Church of Christ is the Successor of Peter, the Bishop of Rome, the Holy Roman Pontiff, the Pope.
The Pope is the Vicar of Christ on Earth. He stands in Christ's place, on Christ's behalf, to shepherd Christ's flock.
Lumen Gentium states:
"[T]he Roman Pontiff, by reason of his office as Vicar of Christ, and as pastor of the entire Church has full, supreme and universal power over the whole Church, a power which he can always exercise unhindered."
Successors of the Apostles, the Bishops of the particular Churches throughout the world form the College of Bishops. Individual bishops have charge of a particular diocese. Together as a group the Bishops may exercise power over the Universal Church by coming together in an ecumenical council (such as Vatican II). However, even ecumenical council's must be recognised and agreed to by the Successor of Peter to be valid. To the extent that the College of Bishops is not united with its head, the Pope, then it has no authority at all.
The lay faithful are those baptised faithful not called by God to ordained ministry within the Church. The laity, being immersed in the world, are called in a special way to bring Christ to the world.
Pope Pius XII said:
"Lay believers are the front line of Church life; for them the Church is the animating principle of human society. Therefore, they in particular ought to have an ever-clearer consciousness not only of belonging to the Church, but of being the Church, that is to say, the community of the faithful on earth under the leadership of the Pope, the common Head, and of the bishops in communion with him. They are the Church."
Although not forming part of the official hierarchy of the Church, certain bishops are granted special status and position within the Church by being elevated to the College of Cardinals. The primary role of the College of Cardinals is to act as special advisors to the Pope and to come together on the death of a Pope to vote for his successor.
The individual Bishop has the primary duty of caring for the faithful in his diocese. It is he who must teach them and shepherd them. However, every country or region now also has an Episcopal Conference (eg, the Australian Catholic Bishops Conference, the National Conference of Catholic Bishops in the United States, the Conférence des Evêon;ques de France). The new Code of Canon Law reserves certain disciplinary decisions to the Episcopal Conference for a region. For example, the general law in the Code of Canon Law is that Catholics must do penance on Fridays by abstaining from eating meat, but it allows the Episcopal Conference for each region to substitute a different penance if they choose (which most appear to have done).
The Episcopal Conferences are not part of the Magisterium of the Church. They have no authority to teach in and of themselves. Their interpretations of doctrine and pronouncements on them are only binding insofar as your own bishop has lent his name to the interpretation or pronouncement.
Cardinal Ratzinger has some interesting things to say about the danger of Episcopal Conferences and their ability to stifle the voices of good bishops in The Ratzinger Report. (Available in the Bookstore)
The Synod of Bishops is a formal body within the Church. It is an advisory body to the Pope that meets in Rome on set occasions. It is largely a creature of the Second Vatican Council and many considered it to be a continuation of the Council: the Bishops coming to Rome to consider important issues and develop Church teaching. Many considered that the Synod of Bishops was an expression of the collegiality of Bishops, a move towards the democratization of the Church and the decentralization and diminution of Papal power. It was considered that the Pope, although theoretically still in charge, would not act contrary to the Synod of Bishops. With that perspective, the Synod of Bishops has proved to be a toothless tiger.
In the sense that it is to assist the Holy Father to understand the needs of the Church throuhgout the world, the Synod has proved to be valuable, and Pope John Paul II certainly holds it in high esteem. But, the Synod really is only an advisory body and the final decision (and teaching) remains with the Holy Father. Only Bishops together in a General Ecumenical Council have the power to teach as a College (like the Second Vatican Council). And even Ecumenical Councils are ultimately subject to the authority of the Holy Father.
It was the Synod of Bishops meeting on the Family that led to the Holy Father's Apostolic Exhortation Familiaris Consortio on the Role of the Family in the Modern World.
More recently, there have been a number of Special Assemblies of the Synod of Bishops from various regions of the World (the Special Assembly for Africa in 1995, the recent Special Assembly for the Americas in December 1997 and next year will be the Special Assembly for Oceania). These Special Assemblies are a special preparation for the Jubilee of the Year 2000.
Although not part of the ordained hierarchy, there also exists within the Church those who are called to live a life consecrated to God. These faithful profess the "evangelical counsels" of charity, chastity, poverty and obedience under permanent vows as their state of life. This may take many forms, including nuns (consecrated virgins), hermits, monks and friars. These faithful live in community with each other according to the rules established for the community and approved by Rome.