The Divine Office ("Liturgy of the Hours")

The expression "Divine Office" signifies etymologically a duty accomplished for God; in virtue of a Divine precept it means, in ecclesiastical language, certain prayers to be recited at fixed hours of the day or night by priests, religious, or clerics, and, in general, by all those obliged by their vocation to fulfill this duty. The Divine Office comprises only the recitation of certain prayers in the Breviary, and does not include the Mass and other liturgical ceremonies. 

Priests as well as deacons aspiring to the priesthood are obliged to fulfill the liturgy of the hours daily in accordance with the proper and approved liturgical books (CIC, Canon 276.3).

The Roman Breviary, Confraternity of Ss. Peter & Paul

Vespers: Its Meaning and Structure - John Paul II Describes the Church's Evening Prayer

Archdiocese of New York Guide to the Liturgy of the Hours

Monks of Adoration Daily Audio

The Liturgy of the Hours Apostolate