Storms All the Way Down
Jupiter's most visually striking feature is its chaotic and turbulent atmosphere. In fact, Jupiter is almost entirely atmosphere: the planet lacks any solid surface, at least as we usually conceive of one. The gas giant's atmosphere consists almost entirely of hydrogen and helium, while ammonia, sulfur, methane and water vapor exist as "trace" elements. While less than 1% of the atmosphere, these trace elements still contribute millions of tones to the mass of the planet.
If we could penetrate the planet's depths, we would see the components of the atmosphere separate into layers. As pressure and temperature rose, the hydrogen would coalesce into a liquid; near the core it would be converted into metallic hydrogen, a substance only just created this year on Earth in a high-pressure laboratory.
It's the mixing, temperature and movement of these gases that create the distinctive bands and whorls we see on Jupiter's "surface". The lighter regions - known as "zones" - form where gas is rising and cooling; the darker areas, called "bands", are where the atmosphere is sinking downwards and warming. Between any zone and its neighboring band is a chaotic front of churning gas, creating spectacular eddies that form into storms. The largest of these storms, the Great Red Spot, is an anticyclone that has been continuously observed since 1830. The storm may have been active since at least 1665, but poor record keeping and gaps in the observational record mean that we cannot be certain that today's Red Spot is the same feature seen almost four centuries ago. We do know that the storm is shrinking: over the last several decades, its energy has been siphoned off by forces unknown, so that it is now merely the size of the Earth.
The Solar System's Vacuum Cleaner
The mass of Jupiter is so enormous that it acts as a kind of cosmic attractor for stray comets and asteroids, which meet their doom in the crushing depths of the atmosphere; a lucky few have been captured and transformed into orbiting moons. This has made Jupiter a kind of "buffer" for our home planet, which might otherwise have been subject to a far more intense celestial bombardment, possibly wiping out attempts at life. Indeed, the presence of a Great Attractor in a planetary system might be a prerequisite for the development of life.
A Chain of Moons
Some of these moons are almost large enough to be considered planets themselves, were they not already orbiting Jupiter. The four original "Galilean" moons - Io, Europa, Ganymede and Calisto - exist in circular orbits around the planet. Io is the most volcanically active body in the solar system, it's interior constantly pushed and pulled by gravitational attraction between its mother planet and other moons. The same tidal forcing is thought to maintain a sub-surface ocean of water on Europa, under an ice shell many kilometers thick.