Jupiter is the largest planet in the solar system and, as such, has the largest number of moons, 67 in total, although it is worth remembering that these are the ones that have been discovered, and there may be more, as new ones are being discovered all the time, with 4 being found in the 5 years or so since 2009.
The largest of these moons are known as the Galilean moons, and were discovered as far back as 1610 by astronomer, Galileo Galilei. Their names- Lo, Europa, Ganymede and Calllisto- are the former lovers of the Greek god, Zeus. These moons are among the largest objects in our solar system, and to put that in perspective, Ganymede, the largest of them, is bigger than the planet Mercury.
Jupiter's many moons differ great in terms of size, shape and time and trajectory of orbit. Some have active volcanoes, whilst others are covered in an icy rocky surface, or have bodies of water beneath their surface.