The Introit is best known in the Tridentine Mass; however, it is part of other Catholic Mass liturgies, and even other Christian liturgies. In Ambrosian chant and Beneventan chant, the counterpart of the Introit is called the "ingressa." In the Sarum rite, it is called the "officium". Many Lutherans also have an introit in their service.
Most Introits are taken from Psalms, though many come from the rest of scripture. Generally they follow the same structure: two to four lines of scripture related to the theme of the feastday or celebration. Most often the choice of scripture passage has something in common with the liturgical readings that will be featured later in that Mass.
The Introit can be either sung or spoken, depending on the formality of the Mass as well as the preferences of the priest and his congregation. Not all Masses have Introits; traditionally the service of Holy Saturday in the Roman Catholic rite does not.
In many cases the Introit also serves another purpose: it gives a name to a particular Mass, based on the first word or phrase of the Introit. Since the Introit for a Mass is different for each day of the Liturgical Calendar (sometimes multiple Masses even exist for a single day, such as Christmas), and unique Masses exist for special functions, it provides a key for determining which order of the Mass is to be performed. This is why a funeral Mass is called a Requiem and the second Mass of the Feast of the Sacred Heart is called the Exordium, for example. As an unusual example, the first Sunday after Easter Sunday, though having no official liturgical name, is colloquially labelled "Quasimodo Sunday" as the first phrase of the Introit is "Quasi modo geniti infantes..." taken from 1 Pet 2:2. Even though Latin is no longer mandatory in the celebration of the Mass, the traditional names remain for purposes of organization of the calendar.
The practice evolved from the singing of a psalm as the priest and ministers approached the altar, sung verses having been part of the celebration of the Mass since earliest times. The Liber Pontificalis claims that the Introit originated by the request of Celestine I, but it was in the reign of Gregory I that the familiar form emerged, and Gregory is popularly believed to have composed many Introits himself; he is in fact not known to have composed any music.
In the musical idiom of Gregorian chant, Introits normally take the form antiphon-verse-antiphon-doxology-antiphon. Introits, like Offertories and Communions, are believed to have evolved from simpler reciting tones. Introit melodies show this musical parentage most clearly, and are often anchored around two reciting notes which may be repeated or percussed. The melodies are mostly neumatic, dominated by neumes with two or three notes per syllable, although syllabic and melismatic passages also occur.
The Introits of Old Roman chant share many similarities with their Gregorian cousins, and often include a repeated extra verse that fell out of use in the Gregorian repertory.
So, for example, in the text used for the third/fourth Lord's Day of Advent, we have the antiphon Rorate Caeli from Isaiah 45:8a:
The Verse and Antiphon from Psalm 18:2
Then the doxology.
Which is followed by the initial Antiphon (Rorate ... Salvatorem)
Appropriate to the season just before the Nativity, a translation: