Holy Orders refers to the ceremony in the Catholic, Anglican, and Orthodox Churches that sets aside ("ordains") persons to lead the church and to celebrate the sacraments. There are three levels of Holy Orders: Bishop, Presbyter, and Deacon. In the Catholic and Eastern Orthodox churches, Holy Orders itself is considered to be a sacrament. In those churches, only males are ordained.
A Deacon is ordained to preach and minister, both in the Liturgy and otherwise. In the United States, for example, the Catholic Bishops have designated Social Justice issues as particularly appropriate for deacons, as they are a link between the clerical and lay states. A Catholic Deacon, with proper permission, can baptize, lead Communion services or assist the priest at the Eucharist, and preside at weddings.
A Presbyter is ordained from among the deacons to administer most of the sacraments. Thus, besides the deacon's ministries mentioned above, a Roman Catholic or Anglican priest (presbyter) can, with permission, preside at the Eucharist, administer Reconciliation and give absolution (assurance of forgiveness of sins), and Anoint the Sick. Under certain circumstances, Roman Catholic priests can also Confirm members of the Church.
Presbyters are usually called "Priests," but (technically speaking) this term refers to both Presbyters and Bishops. In the Catholic Church, both are believed to "take the place of Christ," the High Priest, at the Eucharist. In the Anglican churches, the word "priest" is retained for historical reasons even though the officiant at Communion is not believed to stand in the place of Christ or to "re-present" Christ's sacrifice on the Cross during the Eucharist.
A Bishop has the fullness of orders. Ordained from among the Priests (Monks in the case of Orthodoxy), the bishop can administer any of the sacraments. A bishop is usually assigned to or elected to head a diocese, and sometimes also to a local church.