At Mass, the priest takes bread and wine in his hands -- hands he has put at the service of Christ -- and prays for the Holy Spirit to make them the Body and Blood of our Lord Jesus Christ. The Eucharist is an unbloody re-presentation (not representation) of the sacrifice of Jesus Christ on the cross for us. It is one and the same; Jesus is not sacrificed over and over again, but the priest offers to God that same sacrifice, that same spotless lamb.
Erin Arlinghausen blogged on something Fr. John Corapi said about the Eucharist. It's not just made present to us in that it is presented to us, it is made present to us, rather than past, just as for the Apostles it was made present to them, although the sacrifice on the cross was still to come in the future.