The Liturgy is the Public Prayer of the whole Church. The two chief liturgical actions of the Church are the offering of the EUCHARIST or the MASS, and the DIVINE OFFICE.
In the Mass, the one un-repeatable sacrifice of Jesus Christ on the Cross on Mount Calvary two-thousand years ago is re-presented in an unbloodied manner on the altar so that we may benefit from its effects here and now. The Mass is a Sacrifice and a Sacrament since we are united to the Person of the Living Christ by His spiritual presence amongst us and by partaking of His Eucharistic Body and Blood in Holy Communion.
The Liturgy of the Mass is complemented by the Liturgy of the Divine Office, which is the praise of the Church to God through the consecration of the various Hours of the Day (Dawn, Mid-morning-Noon-Midafternoon, Evening, and the End-of-Day at Night). The Divine Office consists principally of the recitation of the Psalms from the Bible, hymns, lessons from Scripture, and other petitionary prayers. Since the celebration of the Divine Office is correlated to the hours of the each day, the Church's prayer is unceasing throughout the world.
While earth rolls onward into light,
Through all the world her watch is keeping,
And rests not now by day or night
--Hymn for the Night Office