Catholics believe that the Body, Blood, Soul, and Divinity of Jesus Christ — that is, the whole Christ in both his humanity and his divinity — are made present under the appearances of bread and wine in the Eucharist at each Mass (or Divine Liturgy). Catholics also believe that the Eucharist is the same sacrifice which Jesus offered to the Father on Calvary some 2000 years ago: "the victim is one and the same, the same now offering by the ministry of priests, who then offered Himself on the cross, the manner alone of offering being different." (Council of Trent, Session XXII, Chapter 2)
So in the Mass (or Divine Liturgy), the sacrifice of Jesus Christ is being made present, both in its immolation (which took place on the cross in a bloody manner, but which is presented in an unbloody manner via the separate consecration of the bread and wine) and its offering (which takes place eternally in heaven). The sacrifice is made present without crucifying Christ again, without making the crucifixion (which was bloody) present again.
Thoughts? Questions? Comments?