A "Galilean" moon, Europa is Jupiter's sixth satellite. Europa has some similarities to Io: its size and young surface. However, volcanic activity on Europa is nonexistant. Instead, the surface is smooth and covered with lines.
At first, scientists believed that Europa's surface lines might be cracks caused by tectonic plate movements or rifts in the moon's crust. Once high-resolution photos of Europa reached us, however, scientists were confused by the smoothness and lack of topographical features on Europa. Eventually it was decided that Europa must have a hot core like Io, but not as hot. Europa's core needs to let out heat, just like Io's, and it does so by heating the planet's icy undersurface. This heat melts away the ice down deep to create a small ocean around most of the planet. Crust plates float around on this ocean layer, which helps to explain the moon's streaky weather patterns.
Europa, unlike most moons, does have an atmosphere. It is not enough to support life, and it definitely did not come into existence the same way Earth's did (through the life forms themselves!). Speaking of life, though, Europa does have one ingredient which is absolutely essential for life: liquid water. On no other place outside Earth in the Solar System does liquid water exist.
* Photo credit - JPL