What is the Mass?

Good question.  The Mass is the central act of worship for a billion Catholics worldwide.  The Mass is the re-presentation (not representation) of the Last Supper Jesus held with His apostles and His sacrifice on the cross.  The central act of the Mass is the Eucharist where unleavened bread is turned into the Body of Christ and wine becomes His blood.  The Mass fulfills Jesus commandment to "do this in memory of Me."  The Mass, with the Eucharist at its heart, has been the central act of worship for Christians throughout the centuries. 

For more on the Mass try the following:

http://www.newadvent.org/cathen/10006a.htm

http://www.catholic.com/library/sacraments.asp

http://www.scripturecatholic.com/the_eucharist.html