Short answers, pulled from the story.
A shepherd moon is a small natural satellite that clears a gap in a planet's ring material or keeps ring particles confined to a narrow band. Its gravity deflects nearby particles through close proximity or orbital resonances, carving gaps or holding ringlets together.
Goldreich and Tremaine first theorized the existence of shepherd moons in early 1979, based on the unusually thin, sharply gapped rings of Uranus. They proposed that two small, then-undetected satellites might be confining each ring, and Voyager 1 photographed the first shepherd satellites later that same year.
Pan is the shepherd moon that maintains the Encke Gap in Saturn's rings. Saturn's ring system also has Daphnis at the Keeler Gap, Prometheus at the F ring, and Janus and Epimetheus shepherding material within the A ring.
Cordelia and Ophelia shepherd Uranus's narrow epsilon ring from the inside and outside respectively. Both moons orbit well within Uranus's synchronous orbit radius, so tidal deceleration is slowly causing their orbits to decay.
Voyager 2 images showed that Neptune's rings are complete circles with bright clumps, contradicting earlier Earth-based observations that suggested incomplete arcs. The gravitational pull of the shepherd moon Galatea, and possibly other undiscovered shepherd moons, is thought to cause the clumping.
Chariklo, a centaur beyond Neptune, has remarkably well-defined rings suspected to be either very young or held in place by a shepherd moon similar in mass to the rings. Chiron, another centaur, is also thought to have rings similar in form to Chariklo's.