Holy Orders is one of the two sacraments of service. The other is Marriage. Holy Orders is a vocation, a call to holiness and to the mission helping to evangelize the world. Those ordained to Holy Orders, are consecrated in Christ to build up the body of Christ in his Church.
There are three forms of ministry through Holy Orders.
Deacons: ordained ministers to the Catholic Church and consecrated as a sacramental sign to the church and to the world “to serve and not to be served” as Christ served
Priest: ordained and consecrated into the ministry to participate into the mystery of Christ made present through the Eucharist
Bishop: conscerated into the fullness of Christ, as teacher and shepherd of his flock