Phoebe is a roughly spherical Moon that is just 1/15th the size of our Moon with a diameter of 220km. This leads many to think that it is probably just a captured body (i.e. a passing body that has been caught in Saturn's gravitational field).
It orbits at a distance of about 12,952,000 km and completes an orbit in around 550.3 Earth days. It orbits the planet in the opposite direction than the other moons (and most moons in the Solar System) and rotates on its axis once every 9 hours.
Astronomers believe that Phoebe is a captured Centaur. A Centaur is a Kuiper Belt object that has migrated to the inner Solar System. This is a particular interest to astronomers because it is believed that they are as old as the Solar System.