In the Mass, some parts of the rite are to be sung by everyone, and some parts only by the priest, or only by the congregation. Most of these parts of the rite have fixed texts: they do not change from week to week.

There are, however, parts whose text does change from week to week or even from day to day. Some are only sung one day a year. We couldn't expect a congregation with no rehearsal to sing them, so a choir is needed with the appropriate skill and time to study.

These changeable parts of the Mass, called the "proper" parts, include these:

Entrance Chant
Offertory Chant
Communion Chant

There are also "proper" chants for these:
Responsorial Psalm, called the "Gradual Psalm"
Alleluia melody and verse (Or, in Lent, the "Tract" and its verses)

Each of these has the structure of an antiphon with verses.

One book containing these proper chants in Latin for each Sunday Mass is the "Gregorian Missal". http://media.musicasacra.com/books/gregorianmissal-eng.pdf

From newadvent.org:

Schola Cantorum

A place for the teaching and practice of ecclesiastical chant, or a body of singers banded together for the purpose of rendering the music in church. In the primitive Church the singing was done by the clergy, but, in order to set them free from this and enable them to give their attention more to what strictly pertained to their office, trained singers for the musical part of the liturgy were introduced. Pope Hilary (d. 438) is sometimes credited with having inaugurated the first schola cantorum, but it was Gregory the Great, as we are told in his life by John the Deacon, who established the school on a firm basis and endowed it. The house in which the schola was lodged was rebuilt in 844 by Pope Sergius II, who had himself been trained in it, as were also the popes Sergius I, Gregory II, Stephen III, and Paul I. This Roman school furnished the choir at most of the papal functions and was governed by an official called prior scholae cantorum or simply cantor. From Cardinal Thomasi's preface to the twelfth-century Vatican antiphonary, we learn that, amongst his other duties, he had "to point out to each individual, the day before, what responsory he was to sing in the night office". From Rome the institution spread to other parts of the Church. Pepin, the father of Charlemagne, first introduced Roman chanters into France, placing them at Lyons. Charlemagne encouraged the work, and through his influence several other schools were established in his empire. That of Metz became one of the most famous; other well-known ones were at Hirschau, Corbie, and St. Gall. In England the diffusion of the Roman chant was due chiefly to St. Benet Biscop and St. Wilfrid. Several of the cathedrals (e.g. York, Sarum, Hereford, and Worcester) and many of the abbeys (e.g. Glastonbury and Malmesbury) had important scholae cantorum attached to them. The Protestant Reformation put an end to the English schools, while abroad they seem to have died out when paid singers began to be employed in the churches, though perhaps the maîtrise or cathedral choir-school of today may be regarded as their legitimate successor. In monasteries at the present day the name schola cantorum is often applied to certain selected monks whose duty it is to chant the more elaborate portions of the liturgical music, such as the graduals and alleluias at Mass, the rest of the community joining only in the simpler parts. The official in charge of such a schola is usually called the "precentor". In recent times the chief schools of ecclesiastical chant have been at Ratisbon, Mechlin, Einsiedeln, Beuron, and, greatest of all, Solesmes. In these the study of the manuscripts and the work of restoring the traditional chant of the Church have been pursued with much success. The schola of Solesmes was commenced by Dom Guéranger and has been ably carried on by his successors, DD. Pothier and Mocquereau. The latter is precentor at Solesmes (now in the Isle of Wight, England), while the papal commission entrusted with the work of preparing the official Vatican edition of the Chant is presided over by Abbot Pothier. (See PROSPER LOUIS PASCHAL GUERANGER; SOLESMES).

A secondary (IMO) purpose of the choir is to support the congregation’s singing. Of course, if the choir embraces no liturgical “identity” of its own, then singing with the congregation becomes the choir’s primary function … though, at that point, one wonders why not just to disperse the “choir members” among the congregation, which would arguably be more effective toward the end of promoting congregational singing.

I don't think so. It is a novely to think that the choir is supposed to support congregational singing. The congregation has its role and the choir has its own. Just because the congregation has 'lost its way' in what it is supposed to be singing doesn't give license to 'rearrange' the priorities of the schola. The sollution is to FIX the problem with the congregation... not use the choir as a crutch and displace its role and importance.

Wow ! Thank you all,
Ph