An oratorio is a large musical composition. The music is composed for three purposes: for orchestras, for choirs, and for soloist. An oratorio is similar an opera in that both make use of soloists and choirs. In both mediums, strong characters are employed and arias are sung. The difference between an oratorio and an opera is that operas are musical theater, but oratorios are solely concert pieces. Oratorios typically have no interaction with other characters. They do not use props. Costumes are simple, rather than the elaborate ones often seen in operas. The largest difference, however, is that while operas are usually about history or mythology, the oratorios are religious in nature and deal with sacred topics; for this reason, they are often performed in houses of worship. The choice of sacred topic depends on whether the piece is being performed for Protestants or Catholics. Protestant oratorios are usually based on biblical tales; Catholic oratorios are often about the lives of the saints (but also biblical stories). The height of popularity for oratorios occurred in Italy, around the early seventeenth century, as they could be performed during Lent while operas could not, due to the prohibition of “spectacle” during the holy season.
Musically speaking, a “mass” is a choral composition. It is a form of sacred music that puts portions of the Eucharistic liturgy of either the Catholic Church, the Luthern Church, or the Anglican Communion to music. Most masses are performed in Latin, but increasingly, masses are being held in whatever the language is in the country in which they are being performed, even in non-Catholic countries. The Church of England holds mass in English, for example. In these non-Catholic sects, masses are frequently called “Communion Services.” Masses, like oratorios and parts of some operas, can be sung a capella, that is, voice only without musical instrument accompaniment.
An opera differs from oratorios and masses in that, along with singing, it combines musical instruments. Operas are performed in theaters, not churches. Operas are dramatic works that tell stories (called “librettos.” They include costumes and scenery, and sometimes dance, unlike masses and oratorios. The performance are either performed along with an orchestra or with a smaller musical ensemble. There subject matter is typically not religious, but take on historical events, like Claudio Monteverdi’s L'incoronazione di Poppea (The Coronation of Poppea) or mythological ones like Donzenitti’s Fausta.