“The Church draws her life from the Eucharist.” - Pope St. John Paul  
The public worship of the Roman Catholic church is its liturgy, principally the Eucharist, which is also called the Mass.  The Mass is that divine action in which Jesus, through the agency of the human priest, changes the bread and wine into His own Body and Blood and continues through time the offering which He made to God on Calvary—the offering of Himself for mankind.