It begins:**
Preface**. The Eucharistic prayer begins with a thanksgiving offered to God. Indeed, it is thanksgiving that gives the Eucharist its name (Greek, meaning “Thanks”). During the preface, the priest says “The Lord be with you… Lift up your hearts… Let us give thanks to the Lord our God.” After this dialogue, the priest gives thanks to God for his saving acts.
It ends:
Final Doxology and Amen - The Eucharistic prayer comes to a close when the priest gives the final doxology and the people or choir respond, “Amen.” A doxology is a prayer that praises God’s glory. “*Through Him, with Him, in Him, in the Unity of the Holy Spirit, all glory and honor is yours, almighty Father, for ever and ever.” *We respond with “Amen,” and stand. Amen is a Hebrew word used to signify agreement with a prayer and confidence that God will hear it. This Amen is our most important role in the Eucharistic prayer, since we give assent to what the priest has prayed in our behalf. This is why we call it the “Great Amen”.
This is taken from Jimmy Akin’s Mass Appeal.