The Divine Liturgy of St. John Chrysostom is the central act of worship of Byzantine Catholic Christians. This liturgy is shared by all churches of the Byzantine Rite in the Catholic communion. For Roman Catholics, they would call this the Mass. The liturgy is one of the oldest in the Catholic church and pre-dates the Tridentine liturgy by many centuries. Following Byzantine tradition, the liturgy is sung in the vernacular by the priest and the people.
During every liturgy, the sacrifice of the cross is remembered (Gk: anamnesis) through the celebration of the Eucharist. The Eucharist is celebrated with leavened bread (Gk: artos) and wine. After all of the Eucharistic prayers have been said and the Holy Spirit has been called down to change the elements (Gk: epiklesis), the Body and Blood of our Lord Jesus Christ are truly offered to us "for the remission of sins and life everlasting".
Click for the full text of the Divine Liturgy. Byzantines.net also offers a walk through of the Divine Liturgy.