Masses can be a capella, for the human voice alone, or they can be accompanied by instrumental obbligatos up to and including a full orchestra. Sometimes the music in the Mass format was never intended to really be used in a real Mass. Generally, for a composition to be a full Mass, it must contain the following six sections, which together constitute the "ordinary" of the Mass:
The text here is simply: Kyrie eleison; Christe eleison; Kyrie eleison (Κυριε ελεησον; Χριστε ελεησον; Κυριε ελεησον). This is Greek for "Lord have mercy on us; Christ, have mercy on us; Lord, have mercy on us."
This is a celebratory passage praising God and Christ, which sets the following text:
The longest text of the Mass, this is a setting in Latin of the Nicene Creed.
This is a doxology praising the Trinity which begins with the words Sanctus, Sanctus, Sanctus, Domine Deus Sabaoth; pleni sunt coeli et terra gloria tua (Holy, Holy, Holy, Lord God of Hosts; Heaven and earth are full of thy glory). There is also a section that begins with the words Hosanna in excelsis, "Hosanna in the highest."
This is a setting of the Latin words Benedictus qui venit in nomine Domini. (Blessed is he who comes in the name of the Lord).
After this is sung, the Hosanna is usually repeated.
VI. Agnus Dei
A setting of the Latin phrases,
Agnus Dei, qui tollis peccata mundi,
In a liturgical Mass, there are other sections that may be sung, often in Gregorian chant. These sections change with the day and season according to the Church calendar, and are usually not set to music by a composer who wishes to write a Mass. They can, and have been made the subject of motets and other musical compositions, however.
These sections of the Mass as a musical composition have been standard since the Middle Ages; the very earliest Masses may include other parts, and omit some of the standard ones. The first Mass we know of whose composer can be identified was the Mass of Our Lady by Guillaume de Machaut.
The mass as a musical form flourished during the Renaissance, where it served as the principal large-scale form of composition for most composers. Many important masses were composed by Josquin des Prez, William Byrd, and Giovanni Pierluigi da Palestrina, whose Mass for Pope Marcellus is credited with saving polyphony from the censure of the Council of Trent.
After the Renaissance, the mass tended not to be the central genre for any one composer, yet some of the most famous of all musical works of the Baroque, Classical, and Romantic periods are masses. These include the B Minor Mass of Johann Sebastian Bach (who was not a Roman Catholic), Wolfgang Amadeus Mozart's Mass in C minor, the late masses of Joseph Haydn, and Ludwig van Beethoven's Missa Solemnis and Mass in C major.