The Catholic Church has a hierarchy, just as any other structured environment. Having a hierarchy helps the Church lead the faithful at local and increasingly higher levels. The parish is at the most basic level, followed by the diocese, the archdiocese, and then the Church.
The Catholic parish
Practicing Catholics belong to a parish that supports the local church (where they attend Mass every week) and the clergy members who serve the congregation. The head of the parish is the pastor.
The local Catholic parish is often named after a title of the Lord Jesus Christ, such as Blessed Sacrament or Sacred Heart, or a title of the Blessed Virgin Mary, such as Our Lady of Good Counsel or Our Lady of Seven Sorrows, or after one of the saints, such as St. Ann, St. Bernadette, or St. Joseph. The parish is the heart of the diocese because it’s where most Catholics get baptized, go to confession, attend Mass, receive Holy Communion, are confirmed, get married, and are buried from.
The Catholic diocese
The pastor, and every parish priest, answers to the local bishop, who runs the diocese. The local diocese is composed of a collection of local parishes, just like a state is a collection of counties and cities. Many dioceses are comprised of several state counties, and in a few places, the entire state makes up one diocese.
You can think of a local parish like a town or city, and the local pastor is like the mayor. The diocese is like a state or province, and the bishop is like the governor. (The pope is like the prime minister, governing the entire nation, except that he governs the universal Church all over the world.)
The bishop who heads the diocese is not an ambassador of the pope but governs the local diocese as an authentic successor of the apostles, just as the pope governs the universal Church as the successor of St. Peter.
The Catholic archdiocese
An archbishop runs a really large diocese, known as an archdiocese. For example, an archbishop is given authority in each of the following archdioceses: Newark, San Francisco, Denver, Hartford, Miami, St. Louis, and Omaha. Sometimes, the archbishop is also a cardinal. Cardinals spend most of their time working in their archdioceses or in the Vatican.
The Catholic Church
The highest point in the Catholic hierarchy is, of course, the pope, who oversees the Catholic Church and its faithful from his home in the Vatican City, an independent nation situated in Rome.