4 inside Io's orbit. 8 outside Callisto's orbit. The outer 8 probably captured as 2 bodies that broke up to make 8 moons. Galilean Moons Io Europa have rocky compositions. Io - most geologically active object in entire solar system.
Surface looks like a giant pizza.
Essentially no craters because always resurfacing itself. Europa's gravitational tugs won't let it circularize. See fields of water ice with what appears to be fractures in the ice. There is reasonable evidence building that Europa may have oceans beneath the ice - eg. Galileo satellite like to speculate about life in these possible oceans.