Short answers, pulled from the story.
A natural satellite is an astronomical body that orbits a planet, dwarf planet, or small Solar System body, and sometimes another natural satellite. Natural satellites are colloquially called moons, by analogy with the Moon, Earth's natural satellite.
The Solar System contains 455 natural satellites with confirmed orbits, spread across six planetary satellite systems. Seven objects commonly considered dwarf planets, including Pluto, Haumea, Makemake, and Eris, are also known to have natural satellites.
The German astronomer Johannes Kepler first used the term satellite in his 1610 pamphlet Narratio de Observatis a se quatuor Iouis satellitibus erronibus. He derived it from the Latin word satelles, meaning guard, attendant, or companion.
Most regular moons are tidally locked because tidal forces raised by the planet drain energy and slow the moon's rotation until the same side always faces the planet. Exceptions include Saturn's Hyperion, which rotates chaotically because of Titan's gravity.
Io is the most volcanically active body in the Solar System, while Europa, Enceladus, and Triton show ongoing tectonic activity and cryovolcanism. For Io, Europa, and Enceladus, this activity is powered by tidal heating from eccentric orbits close to their giant-planet primaries.
The seven largest natural satellites, each bigger than 2,500 km across, are Jupiter's Galilean moons Ganymede, Callisto, Io, and Europa, plus Saturn's Titan, Earth's Moon, and Neptune's Triton. Ganymede and Titan are larger than the planet Mercury, while Callisto is about the same size.
No subsatellites, or moons orbiting another moon of a planet, are currently known, because a planet's tidal effects would usually make such a system unstable. Possible exceptions include large moons on wide orbits such as Titan, Iapetus, Callisto, and the Moon.