The core of the Liturgy of the Hours is the Old Testament book of Psalms, 150 poems which span the whole range of forms of prayer. Some psalms praise God's goodness and justice, others cry out to Him for help in times of sadness and trouble, others are hymns of thanksgiving. These ancient prayers were recited or chanted in prayer by the Jews at the time of Christ. Probably Jesus and His disciples often prayed the psalms together. Certainly, Jesus often uses phrases from the psalms in the Gospels.
In the earliest centuries of the Church, the first Christians continued the Jewish practice of praying the psalms. In keeping with St. Paul's advise to "pray without ceasing" (1 Thes. 5:17) the early Christians paused when the rose, at mid day, and before going to bed to pray -- often using the psalms as their text. Hippolytus gave the following advice to Christian laymen in his Apostolic Tradition around 200 AD, "If you have a wife, pray the psalms, alternating verses with her. If you have a wife but she is not yet a believer, go apart by yourself and pray alone, and come back to your place with her."
Two thousand years later, our Liturgy of the Hours continues in that tradition. Each hour in the Liturgy of the Hours consists of one or more psalms, each with an antiphon (a short prayer said both before and after the psalm). Each hour also includes a short reading from the Old or New Testament and can include a hymn as well.
The the "major hours" are (which all priests are required to say daily):
Lauds (Morning Prayer) is intended to focus on the theme of resurrection. We remember the Light of the World as we see the first light of day and prepare for our day's work. (General Instruction on the Liturgy of the Hours: 38) It consists of a hymn (optional), two psalms, an Old Testament canticle (a poetic section like a psalm from another book of the Bible), a short reading, and the Canticle of Zechariah (Luke 1:68-79) which prophesies the coming of the Messiah.
Vespers (Evening Prayer) is intended to focus on the theme of thanksgiving and repentance. Looking back on the day, we give thanks for the blessings we enjoyed and ask forgiveness for our shortcomings. (General Instruction on the Liturgy of the Hours: 39) It consists of a hymn (optional), two psalms, a canticle from the New Testament,a short reading, and the Canticle of Mary (or Magnificat) (Luke 1:46-55) in which Mary praises God after the Annunciation.
Matins (The Office of Readings) can be said at any time of day. (In monasteries, the monks wake and say it in the middle of the night as a vigil.) It consists of a hymn (optional), three psalms, and two longer readings: the first from the Bible, and the second from the writings of the Church Fathers or the Saints.
There are also several "little hours":
Compline (Night Prayer) is designed to be said late at night, just before going to bed. It's theme is praying for protection from all fears and evils, and prayer for the dead. It consists of one psalm, a short reading and a canticle from the Gospels.
There are also options for praying several daytime hours (traditionally called Terce, Sext and None), however these are seldom said now except in monasteries and other religious communities. Each consists of a hymn and three psalms and a short reading.
The Liturgy of the Hours is said on a four week cycle, so that if someone prays all the hours for four weeks, he or she will have prayed all 150 psalms.
Whether you say the one hour a day, several hours, or just one hour a couple times a week -- and whether you say them alone or with a group -- the Liturgy of the Hours provides a wonderful means to steap yourself in the ancient prayer of God's people and make the cycle of your day or week holy.
The General Instruction on the Liturgy of the Hours has this to say about the importance of the laity joining in this universal prayer of the Church:
Wherever possible, other groups of the faithful should celebrate the liturgy of the hours communally in church. This especially applies to parishes....
Lay groups gathering for prayer, apostolic work, or any other reason are encouraged to fulfill the Church's duty, by celebrating part of the liturgy of the hours. The laity must learn above all how in the liturgy they are adoring God the Father in spirit and in truth; they should bear in mind that through public worship and prayer they reach all humanity and can contribute significantly to the salvation of the whole world. (GILH, 21,27)