Hail Mary
When praying the Rosary, Catholics pray ten sets of Hail Marys.  Before each set, they pray one Our Father (also called the Pater Noster or The Lord's Prayer).  After each set of Hail Marys, they pray one 'Glory Be' (Gloria Patri).  One set of these prayers is called a "decade."  Usually, Catholics say five decades when they pray the Rosary.