Io is the innermost of the four planet-sized "Galilean" moons of Jupiter. The Galilean moons are significant because when Galileo Galilei spotted them with his telescope in 1610, he showed that they orbit Jupiter and not Earth, which led the way toward disproving the "geocentric" or Earth-centered model of the universe.
Another fact that stands out about Io is its volcanic activity. The Voyager missions observed Io and noticed several active, erupting volcanoes on its surface. This marked the first time that active volcanoes had been discovered off of Earth. Apparently, the volcanic activity is caused by Io's orbit, which passes very close to Europa and Ganymede. The gravitational pull of the two other moons causes Io's shape to change, which in turn causes it to grow hotter. The heat melts the inside of the moon, causing lava to explode onto the surface. This lava is constantly changing and reshaping Io's surface. Io also gains heat by passing close to Jupiter's magnetic field, which produces large amounts of electricity.
The small number of craters on Io's surface suggests that Io is a relatively young moon. Water is also scarce if existent at all, mostly because the extreme heat on the surface has evaporated any that was once there.
* Photo credit - NSSDC Planetary Image Catalog