A Brief History of the Rosary
The concept of counting prayers using beads has been around almost as long as Christianity itself; There are records of people using beads or pebbles to count their prayers leading back to the 3rd century AD. The reason for counting prayers varied - sometimes it was a penance, and sometimes it was simply a promise to pray a given number of prayers each day. But the fact remains that even the earliest church fathers, like
Paul of Thebes and
St. Jerome, were making use of prayer beads.
However, these were not
Rosary beads just yet. Prior to the 13th century, the most common use of prayer beads was to count how many Our Father prayers someone had recited. This led to them being known as Paternoster Beads, after the prayer.
This changed sometime in the late 12th and early 13th century. Monestaries had taken up the practice of praying the entire Book of Psalms - 150 prayers - each day. Those who were illiterate were allowed to say 150 Our Father prayers instead, thus connecting the use of prayer beads with the Psalms. During this time, the was a shift in how parts of the Old Testament were interpreted. Many parts, including the Psalms, were now being viewed with a relation to Mary.
But, while we now had the prayer beads connected to Mary through the Psalms, we did not yet have the Rosary.
The catylist for the creation of the first Rosaries came in the form of a violent conflict with a heretical sect of Christianity. Known as Catharism, this sect rejected the Trinity, believing instead that the god of the Old Testament was a different (evil) deity from the (good) deity of the New Testament. They also held other questionable beliefs, but things took a turn when a papal legate was murdered.
One of the men who rose up to challenge the Cathars and bring them back to Christianity was Dominic de Guzman, a priest and skilled teacher. But even with his best efforts, he could not convince the Cathars of their error. In 1208 he retreated into a forest in France, where he prayed for help with his mission.
After three days of prayer and fasting, he was visited by Mary herself, who explained how to use the Paternoster beads to evangelize to the Cathars. This changed the beads from a tool of worship into a tool for teaching, and with the new use came their new name: the Rosary.
If you are interested in more details about the Rosary or its history, then I'd suggest reading
Champions of the Rosary by Donald H. Calloway.