The four largest moons of Jupiter are known as the Galilean moons, due to their being observed by the famous astronomer Galileo. They are truly large objects, with Europa being as large as Earth’s moon, and Ganymede being the largest moon in the solar system (in fact, it’s larger than both Mercury and Pluto). Io is also famous for being the most volcanically active body in the solar system, due to the gravitational tidal forces it experiences which heat the interior of the planet.

Other Resources

The Astronomy Picture of the Day has a great page discussing the Galilean moons.