Original: The four largest moons, known as the "Galilean moons", are Io, Europa, Ganymede and Callisto.
Simplified: The four main moons are Io, Europa, Ganymede and Callisto. These four are called the Galilean moons. He was the first person to see Jupiter's moons Io, Europa, Ganymede and Callisto. Along with Io, Europa and Callisto, Ganymede is one of the Galilean moons, the four biggest moons of Jupiter.