Operas and oratorios are both large-scale works for voices and orchestra; both involve a narrative set to music.
An opera is a secular play or drama set to music. They can be mythological (Greek/Roman), or "modern". Operas are staged, and acted and sung by solo singers and chorus in costume.
An oratorio is always a sacred work, for example Handel's Messiah or Haydn's Creation. They generally use Biblical text, often directly from the source, and there is no acting or staging. Oratorios were originally performed during Lent.
Operas and oratorios originated in the early Baroque period, and thus tend to be performed by small orchestras.
Oratorios were performed in churches due to their religious nature, however they were also commonly performed in theatres and concert halls. They may be performed as public concerts, or as private soirées for the wealthy.
Types of Pieces
An overture is the introduction to an opera or oratorio, situated right at the beginning.
An aria is a solo song for voice accompanied by orchestra. Arias normally have a memorable melody, but this can be highly decorative and melodically embellished. The music is more important than the text.
Traditionally, arias were an opportunity for the singer to show off their vocal skill, so they often have long melodic phrases with stretched-out melismatic passages.
Narratively, arias convey emotion and show what the character is thinking or feeling.
A da capo aria is in ternary form, meaning it is composed in three sections.
The first section is a complete musical entity, ending in the tonic key, and could be sung alone.
The second section contrasts with the first in its musical key, texture, mood and sometimes also tempo. It usually modulates to the dominant or relative minor.
The third section was usually not written out by the composer, who would instead specify the direction "Da Capo", meaning that the first section should be repeated in full.
A recitative is a speech-like song, where the text is more important than the musical content. It is often syllabic so that atttention is drawn to the words, and the melody is often broken up into small phrases. The orchestral accompaniment is usually very simple, with held or repeated chords, pedals, and a homophonic texture.
Narratively, recitatives focus on telling and advancing the story
A chorus is a piece where the whole choir (usually SATB) performs together. It can contain a variety of musical textures, e.g. homophonic and/or polyphonic.