A shepherd moon is a small natural satellite that clears a gap in planetary-ring material or keeps particles within a ring contained. The name is a result of the fact they limit the "herd" of the ring particles as a shepherd.

Due to their gravitational effect, they pick up particles and deflect them from their original orbits through orbital resonance. This causes gaps in the ring system, such as the particularly striking Cassini Division, as well as other characteristic bands, or strange "twisted" deformation of rings.

Rings Theme:
The rings of Saturn are the most extensive planetary ring system of any planet in the Solar System. They consist of countless small particles, ranging from μm to m in size,[1] that orbit about Saturn. The ring particles are made almost entirely of water ice, with a trace component of rocky material. There is still no consensus as to their mechanism of formation; some features of the rings suggest a relatively recent origin, but theoretical models indicate they are likely to have formed early in the Solar System's history.

Prometheus:
Prometheus was discovered in 1980 from photos taken by the Voyager 1 probe. It was named after Prometheus, a Titan in Greek mythology. Prometheus is extremely elongated, measuring approximately 136 km × 79 km × 59 km (85 mi × 49 mi × 37 mi). It has several ridges and valleys and a number of impact craters of about 20 km (12 mi) diameter are visible. From its very low density and relatively high albedo, it is likely that Prometheus is a very porous icy body.

Daphnis:
Daphnis was discovered by the Cassini Imaging Science Team on May 6, 2005. Daphnis orbits within the Keeler gap in Saturn's rings. As it orbits, it creates gravitational ripples on the edges of the gap as ring particles are attracted toward the moon and then fall back down toward the ring. The moon was discovered to be an irregularly-shaped object with a mostly smooth surface, a few craters, and an equatorial ridge. The moon was named after Daphnis, a shepherd, pipes player, and pastoral poet in Greek mythology. He was descendant of the Titans,

Pan:
Pan is a small, walnut-shaped moon approximately 35 kilometres across and 23 km wide that orbits within the Encke Gap in Saturn's A Ring. Pan is a ring shepherd and is responsible for keeping the Encke Gap free of ring particles. It was discovered by Mark R. Showalter in 1990 from analysis of old Voyager 2 probe photos. The moon was named after the mythological Pan, who was (among other things) the god of shepherds.

Janus:
Janus is extensively cratered with several craters larger than 30 km, but has few linear features. Janus has a very low density and relatively high albedo, meaning that it is likely very icy and porous. It occupies an orbit within the rings with the moon Epimetheus. It is named after the two-faced Roman god Janus.

Epimetheus:
It is named after the mythological Epimetheus, brother of Prometheus. Janus's mean orbital radius from Saturn is only 50 km less than that of Epimetheus, a distance smaller than either moon's mean radius. As the inner moon catches up to the outer moon, their mutual gravitational attraction increases the inner moon's momentum and decreases that of the outer moon. This added momentum means that the inner moon's distance from Saturn and orbital period are increased, and the outer moon's are decreased. The timing and magnitude of the momentum exchange is such that every 4 years the moons effectively swap orbits, never approaching closer than about 10,000 km.

(Source: Wikipedia)

This very long tune is supposed to represent Cassini's travel through Saturn's ring system with fragments of the Ring Theme popping up between encounters with the shepherd moons, each of which has a different musical characteristic, but with closely linked chord progressions and elements of the ring theme.