The practice of using knotted prayer ropes in Christianity dates back to the Desert Fathers in the 3rd and early 4th centuries. These ropes were used as an aid for monks who recited the 150 psalms daily. This practice spread to the laity, who often substituted Our Fathers for the psalms as most people at this time could not read. Eventually, the knotted ropes were replaced with prayer beads, referred to as paternosters, after the first two words of the Our Father in Latin.
The rosary was established as an official devotion in the Catholic Church by another Dominican, Pope Pius V, in 1569. It was at this time that the original 15 mysteries (Joyful, Sorrowful and Glorious) were established. From this time, until the early 20th century, the rosary remained unchanged. Further promotion of the rosary occurred during the papacy of the "Rosary Pope," Pope Leo XIII, who wrote twelve encyclicals and five Apostolic Letters, including promoting daily recitation of the rosary and establishing October as the month of the rosary.
In 1917, the Virgin Mary appeared to three children at Fatima, Portugal and urged that the Fatima Prayer be added at the end of each decade of the rosary. The Blessed Virgin also urged that the rosary be recited for the conversion of Russia and the end of war. The final change to the rosary was made in 2002, when Pope St. John Paul II instituted the Luminous Mysteries, bringing the total number of mysteries to twenty.
Throughout our history, the rosary has served as a popular devotion among Catholics of all ages. The simplicity and beauty of this prayer has provided a source of great comfort to the faithful. Through its recitation, we are able to draw closer to God and meditate upon the mysteries of Christ's life, death and resurrection. As the theologian Romano Guardini stated, the rosary provides a "participation in the life of Mary, whose focus was Christ." For us, the rosary is more than just a set of beads. It is a way to share in Christ's passion and, in some small way, further unite ourselves with the mystical Body of Christ.
Source: Wikipedia