Jupiter, named after the King of the Roman Gods and ruler of Mount Olympus, is very, very large. Its mass is about 318 times that of Earth, and its volume is over 1,330 times greater. It has more mass than the other eight planets combined. It is so large and luminous that it has been known since prehistoric times.
Jupiter has a very low density because it is made mostly from light gases such as hydrogen and helium. The swirls of color that can be seen on Jupiter are actually concentrations of gas called cloud belts. Jupiter spins on its axis very quickly, as do the other giant planets. However, Jupiter doesn't spin in one synchronized motion. Like the Sun, it rotates differentially. It takes longer for certain spots on the planet to revolve around Jupiter's axis than at other points.
Jupiter's most noticeable geographic feature is undoubtedly its Great Red Spot. The Spot is not a mountain or any other fixed object. It is a region of high-pressure, cold gases spiraling counter-clockwise. It looks almost like a hurricane from a close view. However, the Spot stays intact much longer than any hurricane. The Great Red Spot's motion has been monitored almost constantly for hundreds of years.
Jupiter's interior is very hot. The heat comes from the fact that Jupiter is actually shrinking in size. As the planet compresses, heat energy is released. Jupiter sends out more energy than it absorbs from the Sun.
Contrary to expectations, Jupiter has a system of very faint rings. The particles in them tend to drift away, so new pieces of rock must constantly replenish the rings. On average, Jupiter's rings are made of much smaller rocks than Saturn's.
There is a large magnetic field around Jupiter. This field is not circular; it's like a giant windsock pointed away from the sun. Interactions with solar winds cause changes in the size of the magnetosphere. Seven of Jupiter's moons reside inside the boundaries of the magnetosphere.
Jupiter has sixteen known satellites. The four largest are Io, Europa, Ganymede and Callisto. These four moons are called the Galilean moons for their discoverer, Galileo Galilei. All of the Galilean moons except Europa are bigger than the Earth's Moon in diameter. Ganymede, the largest of all the moons, is bigger than the planet Mercury. The existence of so many moons has enabled us to observe their orbits to more accurately calculate data on Jupiter's mass and gravitational field. Much of what we know today about Jupiter, as well as the other outer planets, comes from studying moons.
Jupiter: The Planet - FAQ & references
The Planet Jupiter - Data
The Planet Jupiter - Lots of detailed info