Plainchant
Though the church fathers were ambivalent about the place of music in a moral life, music adorned the liturgy of the church as far back as we can trace. One of our early witnesses to plainchant practice with the nun Egeria whose account of her pilgrimage to Jerusalem from ca. 400 A.D. includes descriptions of psalms and singing. Over the next several centuries, the liturgy continued to develop until it took on the format for the mass and office that is familiar to the modern-day student of the medieval church.
This liturgical practice resulted in part from the religious reforms of Charlemagne (ca. 742-814), who drew on the resources of the church in his attempts to unify his empire. He replaced the disparate local and regional varieties of plainchant (such as Gallican, Mozarabic, and the like) with a single practice. According to Charlemagne's biographers, he wisely decided to send to 'the source,' that is to Rome, for the authoritative versions of chant. The resultant liturgical practice--in fact, a combination of Frankish and Roman elements--is commonly known as Gregorian chant, though recent research has shown that the Pope Gregory involved in the creation of the liturgy was likely Pope Gregory II (r. 715-731), rather than Pope Gregory I (r. 590-604) (see McKinnon, pp. 102ff).
The music of the church can be divided into chants for the mass, which combines a celebration of the Word of God and of the Eucharist, and those for the office, a daily cycle of services involving psalms and prayers, though the requiem mass differs from the daily mass in structure, and various processions are technically paraliturgical. Texts which change every day are called proper, while stable texts which repeat over most of the church year (such as the Kyrie, Gloria, Credo, Sanctus, and Agnus Dei) are called ordinary. Chant melodies range from a simple recitational style involving a single pitch (perhaps with some inflections to provide punctuation), through the straightforward chants which any member of the choir could sing, to the extremely elaborate soloistic chants. Melodies are also classified by how many notes there are per syllable: syllabic melodies have one note per syllable; neumatic melodies generally have two to five notes per syllable; and melismatic melodies have elaborate runs of six or more notes decorating several syllables over the course of the piece. Every service has a mixture of these styles, providing dramatic impetus to the liturgical action.
As Carolingian cantors and their successors attempted to grapple with the importation of nearly four thousand chants for the church year, they developed systems for organizing the musical materials involved. The pressures of memorization supported, and perhaps instigated, the development of a notational system, though the familiar square notation of most surviving chant leaves and most modern-day chant books did not develop fully until the late twelfth century. It also encouraged the development of the system of church modes, which classify chants by their range, their final (the central pitch of the melody where the tune usually ends), and their melodic idiom. The spread of liturgical books, with or without notation, likewise helped to regularize liturgical practice across the realm.
The medieval liturgy has been reconstructed in large part due to the efforts of the Benedictine monks of Solesmes Abbey in France, who have issued facsimiles of early chant manuscripts and compiled editions based on those early sources, including the Liber Usualis, which contains chants, prayers and readings for important services throughout the church year. The Latin liturgy itself, however, has been out of favor since Vatican II (1962-65).
 

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