The Liturgy of the Hours, also called the Divine Office, is part of the public prayer in the Roman Catholic liturgy. As with all liturgy, it involves Christ praying to His Heavenly Father joined by members of His Mystical Body, the Church. It's a collection of Psalms, antiphons, prayers, biblical and spiritual readings and hymns assembled into what's called the Breviary.
The Breviary is a shortened version of the monks' prayer books that originated in the 11th century for use by itinerant friars. The prayers of the monks, in turn, were adapted primarily from Psalms and Scripture passages recited by the Jews prior to Christ. These prayers were traditionally broken into seven segments to be said throughout the day in what's called the canonical hours.
In 1970, the Breviary was revised by Bd. Pope Paul VI with his apostolic constitution Laudis Canticum: what was called Matins prior to 1970 is now called Office of Readings; what was called Lauds is now called Morning Prayer; what was called Terce, Sext and None are now called Daytime Prayer (Mid-Morning, Mid-Day, Mid-Afternoon); what was called Vespers is now called Evening Prayer; and what was called Compline is now called Night Prayer.
The Church in Canon 276 of the Code of Canon Law mandates that clerics are "obliged to carry out the liturgy of the hours daily." Canon 1174 extends this mandate to religious "according to the norm of their constitutions." It further says the laity "are also earnestly invited to participate in the Liturgy of the Hours as an action of the Church."
Priests and religious typically use the four-volume breviary, which is divided into the four liturgical seasons of Advent, Christmas, Lent plus Easter and Ordinary Time. A single volume version, called Christian Prayer, is an abbreviation of the four-volume set that's often used by the laity, who want to participate in this efficacious public worship of God.
Grasp the spiritual value of public worship in Church Militant's Premium show The Download—Praying With the Church.