The Rosary is the most famous and most popular chaplet that we have as Catholics. A chaplet is a prayer devotion that commonly utilizes beads, it comes from a French word for "wreath" or "crown".
While the structure and devotion of the Rosary has its roots with the "desert fathers", it is St. Dominic who is historically credited with giving the structured gift of the Rosary to the greater Church.
In the Rosary we meditate on the life of Christ, gazing upon Him through the lens and window of Mary's soul. Since she is our Blessed Mother, gifted to us from His Cross (Jn. 19:27), it makes perfect sense that we would seek to look upon Christ in such an intimate and sinless way, as she does.
The Rosary draws us into more perfect discipleship, and focuses our prayer on the Trinity.