In the Catholic Church bishops are the successors of the Apostles as shepherds of the flock entrusted to their care. They are to be teachers of doctrine, ministers of worship, and holders of office in church government. A Diocesan Bishop [e.g. the Bishop of Arundel & Brighton] is entrusted with the care of the Catholic faithful within the boundaries of that diocese. He is also to foster good relations with other Christians, people of other faiths, civic authorities, and indeed all people of goodwill.
Through his bond of communion with the Bishop of Rome, a bishop shares with his fellow bishops in a care for the whole universal church. A Bishop also becomes a member of an Episcopal Conference [e.g. England & Wales] and thus plays a part in the development of Catholic life in his region or country.
Through the sacrament of Holy Orders the bishop has a special bond with his clergy [priests and deacons]. They are to be his co-workers in the service of the People of God. The bishop is also assisted in the government of the diocese by the diocesan curia, which consists not only of clergy, but also of religious and laity with special skills and competence in various areas of the life of the Church.
To be appointed a bishop a man must be outstanding in "strong faith, good morals, piety, zeal for souls, wisdom, prudence, and other human virtues". He must be at least 35 years old and have been a priest for at least five years. For England and Wales bishops are appointed by the Pope on the basis of a 'terna' [shortlist of three names] drawn up by the Apostolic Nuncio to Great Britain. In drawing up the 'terna', the Nuncio consults widely amongst other bishops, and also clergy, religious, and laity from within the diocese.
After an appointment is announced the chosen man must prepare himself for episcopal ordination which will happen within three months of his appointment being confirmed. For a diocesan bishop this will more than likely take place in his cathedral church. It is through the outpouring of the Holy Spirit in episcopal ordination that one becomes a bishop, and it is only after his ordination that the new Bishop begins his duties.
The above is a synthesis of the 1983 Code of Canon Law: Canons 375-402
