The word "liturgy" comes from the Greek work "liturgos,"  meaning action.  It is the term used to define the structure  of Christian worship.  For the church of the Middle Ages, this was the Mass (from the Latin word "mensa" or table, a  reference to the sacred meal institutled by Christ at the  Last Supper).  The structure of the Catholic Mass of today  is the same, although certain elements have been  reconstitued. 

The major divisions of the Mass
The Introit (the procession into the church)
The Kyrie (from the prayer "Kyrie eleison" or "Lord, have mercy," the only Greek still used in the Western eucharistic liturgy 
The Gloria (from the angel's hymn of the nativity "Gloria in excelsis Deo" --"Glory to God in the Highest) 
The Credo, or Creed (the statement of belief) 
The Offertory (the presentation of the bread and wine) 
The Consecration (in which the offerings are Transubstantiated into the Body and Blood of Christ) 
The Sanctus ("Holy, Holy, Holy..."), which invokes the Trinity 
The Agnus Dei or "Lamb of God" 

Between the "Gloria" and the Creed, portions of the Old and the New Testaments are read, and a homily is delivered by one of the priests.  In addition to the official prayers described above, there are also hymns appropriate to the season or to the saint or festival be celebrated that day.  The singing of hymns is a legacy from Jewish worship, and some of the church's hymns date from the earliest Christian centuries.