The four moons that Galileo saw when he pointed his telescope at Jupiter are now known as "the Galilean satellites." They are Jupiter's four largest moons, all easily seen with even a small telescope. These moons have now been visited by several NASA space probes which have revealed them to be fascinating worlds. In order of distance from Jupiter, they are:
the closest to Jupiter, Io is a moon caught in a constant tug-of-war between the giant planet and the other Galilean satellites. As a result, Jupiter's gravity produces a constant squeezing of Io, heating its interior. Io is the only body in the solar system other than the Earth on which active volcanos exist. These volcanos spew sulfur and sulfur dioxide, giving the moon a colorful surface of reds, oranges, and white.
Next furthest out is Europa. Being further from Jupiter, Europa is not as heated by Jupiter's pull as Io, and its surface is covered in ice layer a few kilometers thick. However, below that ice is thought to be a water ocean a few hundred kilometers deep. The motion of the ice layer on top of this ocean results in deep fractures in the surface ice which show up as lines crossing the surface.
Ganymede is the largest of the Galilean satellites, larger than our own Moon and even larger than the planet Mercury. Many impact craters can be seen on Ganymede, a sign that, unlike Io and Europa, Ganymede's surface is not changing much. However, the surface does show grooves and cracks, suggesting that at one point in its history it was more active.
The most distant Galilean moon, Callisto is covered with impact craters. Because it is so far from Jupiter, Jupiter's gravitational pull does very little to heat Callisto, making it essentially a solid, unchanging ball of ice. The many old craters and lack of cracks or grooves on its surface indicate that very little has changed on the surface of Callisto since it formed.