Holy Orders is the sacrament through which the mission entrusted by Christ to his apostles continues to be exercised in the Church until the end of time: thus it is the sacrament of apostolic ministry. It includes three degrees: episcopate, presbyterate, and diaconate. (Article 6 in the Catechism of the Catholic Church)
The episcopate ordination confers God's grace on a bishop who receives the offices of sanctifying, teaching and ruling as Christ's representative. Pastoral care of a particular flock is entrusted to each bishop.
Priests are co-workers of the bishops. They shepherd the faithful, preside at divine worhip - the Mass, preach the Gospel and are the priests of the New Testament.
Deacons are concecrated to serve. They assist the priests in the celebration of the Eucharist, preside over and bless marriages, baptize, proclaim the Gospel and preach, preside at funerals and participate in ministries of charity. Permanent deacons may be married.