The first part of the Mass is a kind of introductory service, made up of chants, prayers and lessons (i.e. readings from Holy Scripture) -- namely, the Introit, the Kyrie, the Collect, the Epistle or Lesson, and the Gospel. On certain days the Gloria and the Nicene Creed are added.
This first part of the Mass is called the Mass of the Catechumens, while the remaining part is called the Mass of the Faithful. These names have their origin in the discipline of the early Church. In the first ages of Christianity, persons desiring to become Christians were obliged to undergo a course of instructions preparatory to baptism. They were called "catechumens," a Greek word meaning "one whose is being instructed." Catechumens, not yet fully initiated in the teachings and practices of Christianity, were customarily dismissed before the Offertory.
Likewise public sinners who had not yet been absolved were ordered to leave the church before the Offertory. The Sacrifice of the Mass was considered too holy for the presence of notorious sinners; likewise, it was thought to be too mysterious for catechumens. Only those who were baptized, -- "the Faithful" -- could take part in the actual Eucharistic Sacrifice. The Church, during the course of centuries, modified her discipline in this regard, and all are now permitted to remain.
The Mass is one continuous action, reproducing in a mysterious way the Life, Passion and Death of Jesus Christ. The structure of the Mass is as follows:
§ The Preparation – beginning with the Prayers at the Foot of the Altar, the Introit, Kyrie and Gloria.
§ The Instruction -- including the Collect, the Epistle, Gradual, Alleluia, (or Tract, and on certain feasts the Sequence), the Gospel (usually followed by a sermon), and the Credo.
§ The Offertory -- which includes the Offertory antiphon, the offering of bread, the pouring of water and wine into the chalice, the offering of the chalice, the washing of the hands, the prayer to the Blessed Trinity, the "Orate fratres" and the Secret.
§ The Consecration -- including the Preface and the Canon of the Mass, embracing the prayer "Te igitur," the Memento of the living, the Communicantes and the other two prayers before the Consecration and Elevation, the three prayers after the Consecration, the Commemoration for the Dead, the "Nobis quoque peccatoribus" and the Minor Elevation.
§ The Communion -- including the Pater Noster, the Libera, the Agnus Dei, the three prayers before the Communion, the "Domine non sum dignus," and the Communion of the Priest and the Faithful.
§ The Thanksgiving--which includes the Communion antiphon, the Post- communion prayer, the "Ite missa est," and the Last Gospel.