The Office contains all the hours of worship throughout the day: Matins, Lauds, Prime, Terce, Sext, None, Vespers, and Compline. The Front Matter contains an introduction as well as a table of contents and the Kalendar. The common parts of the Office, used on a daily or weekly basis, are found in the Psalter. The proper parts pertaining to the weeks and seasons of the year are found in the Temporale. The proper parts pertaining to saints days and special feasts are found in the Sanctorale. The Martirology contains a digest of the saints remembered on each day of the year. The Tonale explains the modal theory used in Gregorian psalmody.
The Office is a highly complex but incredibly rich system of daily prayer. See Praying the Office for suggestions on how to approach the material.