Mary, I should have used the less "technical" term of Entrance Chant (introitus
means "entrance"). Forgive this non-native speaker of English. Every Mass - in both forms, Ordinary and Extraordinary - has its own entrance chant as per Roman Missal and it is supposed to be sung at the beginning of the Mass when the priest approaches the altar.
From the GIRM: http://www.vatican.va/roman_curia/congr ... le_en.html
47. After the people have gathered, the Entrance chant begins as the priest enters with the deacon and ministers. The purpose of this chant is to open the celebration, foster the unity of those who have been gathered, introduce their thoughts to the mystery of the liturgical season or festivity, and accompany the procession of the priest and ministers.
48. The singing at this time is done either alternately by the choir and the people or in a similar way by the cantor and the people, or entirely by the people, or by the choir alone. In the dioceses of the United States of America there are four options for the Entrance Chant: (1) the antiphon from The Roman Missal or the Psalm from the Roman Gradual as set to music there or in another musical setting; (2) the seasonal antiphon and Psalm of the Simple Gradual; (3) a song from another collection of psalms and antiphons, approved by the Conference of Bishops or the diocesan Bishop, including psalms arranged in responsorial or metrical forms; (4) a suitable liturgical song similarly approved by the Conference of Bishops or the diocesan Bishop.
If there is no singing at the entrance, the antiphon in the Missal is recited either by the faithful, or by some of them, or by a lector; otherwise, it is recited by the priest himself, who may even adapt it as an introductory explanation (cf. no. 31).
The Extraordinary Form has rules that are somewhat stricter but the idea is the same. Note that all of the above means that the people should know what the Roman Gradual and the Simple Gradual are and be able to sing from them, as per Vatican II's Constitution Sacrosanctum Concilium
Maybe this thread could interest you: viewtopic.php?f=54&t=65133
I am trying to present the "by the book" versions of the Entrance Chants of the Sundays of the liturgical year