In Catholic usage, a chaplet is a prayer rule. As 1ke has stated, the rosary itself is a chaplet.
Different saints over the ages have had particular prayer rules of their own, and these have been passed down to us for our benefit. We can pray with the saints!
The origins of many chaplets are legendary. More than likely, some of the popular chaplets are not actually prayer rules of the saints they are named after, but that would be difficult to determine. They might just be the prayer rule of a less well known person who named them after their own favorite saint.
Outside of specifically Catholic usage a chaplet is a wreath, or ring of flowers or jewels. I can see how the term could be applied to a ring of prayers!
This site lists some popular chaplets, you could also devise your own, in which case you might like to custom-make a string of beads to match out of a rosary making kit.
My favorite standard chaplet is that of Saint Patrick.