Io is the fourth largest moon in the solar system, about the same size as Earth's. But, whereas Earth's moon (like most) is a boring ball of dirt, Io is bat guano insane, with over 400 volcanoes spewing plumes of material from its molten core as high as 500 km into space, creating a thin atmosphere of sulphur which disperses, due to Io's low gravity.
The volcanoes were first noticed by a navigation engineer named Linda Morabito when she was analyzing images sent from Voyager 1. It is also covered with mountains (most tectonic and not volcanic), some higher than any on Earth. It's also highly radioactive. And as pockmarked and hard to look at as it is, it has no known impact craters. Io remains difficult to look at for dermatosiophobes like myself. If you also have this probelm, maybe it will help to compare it to a moldy fruit.
It was first discovered in 1610 by Galileo Bonaiuti de' Galilei, an astronomer curiously referred to, in most cases, by his first name (like Bjork, Sadam, Lawrence, Madonna and Prince) -- a fact which I find fascinating. It's not as if Galileo is an overly common family name. Though named "Io" by Simon Marius in 1614, the moon was usually referred to as Jupiter I until the mid-20th century. Marius claimed to have discovered Io, in fact, a week before Galilei.