Short answers, pulled from the story.
The Moon was first recognized as a natural satellite in 1610 when Galileo Galilei discovered four objects orbiting Jupiter. This discovery shifted human understanding of celestial bodies from the earlier view that the Moon was a planet in its own right.
Johannes Kepler derived the word satellite from the Latin satelles meaning guard, attendant, or companion in 1610. This terminology allowed astronomers to distinguish orbiting bodies from the planets they circled.
Triton is considered unique because it orbits Neptune in a retrograde direction suggesting it was a captured dwarf planet from the Kuiper Belt. This unusual motion indicates it was not born in the Neptunian system but was snared by Neptune's gravity early in the solar system's history.
Volcanic activity on Io is powered by tidal heating generated by the gravitational pull of Jupiter and the resonant influence of the other Galilean moons. This constant flexing creates immense friction and heat that melts the interior rock and drives hundreds of active volcanoes.
Europa, Enceladus, Titan, and Ganymede are all thought to harbor subsurface oceans of liquid water beneath their icy crusts. Enceladus has been observed ejecting massive plumes of water vapor and ice particles from its south polar region providing direct evidence of these hidden depths.
The first artificial satellite Sputnik was launched in 1957. This event created a need for new terminology to distinguish between man-made objects and natural bodies leading to the modern use of the term natural satellite.