Chaplets, Rosaries, & Prayer Beads
Strictly speaking, all rosaries are chaplets ~ both are names for the beads we use to count prayers, and for the prayers themselves. Still, when we say ‘rosary’ most people think of the five or fifteen decade Catholic Dominican rosary, and use ‘chaplet’ or ‘prayer beads’ for all the other specialized prayers and patterns of beads.
Most often, chaplets are created for use in a specific devotion. Sometimes they are created in response to a vision, or to commemorate a saint. They are almost always widely used long before they are formally recognized, sometimes for decades or even centuries.