Jupiter has 63 known satellites (as of Feb 2004): the four large Galilean moons, 34 smaller named ones, plus many more small ones discovered recently but not yet named. Here is a picture showing the orbits of some of Jupiter's many moons:
Jupiter is one of the "gas giant" planets. The gas giants are planets in our solar system that do not have solid surfaces, their atmospheres simply get denser with depth. What we see when looking at these planets is the tops of clouds high in their atmospheres. Jupiter is about 90% hydrogen and 10% helium with traces of methane, water, ammonia and "rock". This is very close to the composition of the primordial Solar Nebula from which the entire solar system was formed.
The info we know about the insides of Jupiter, like Mars, is mostly educated guesswork, based on some basic facts that we can see from observing it with our telescopes and also information we can gather from studying the way gases behave in a laboratory. Jupiter probably has a core of rocky material amounting to something like 10 to 15 Earth-masses. Above the core lies the main bulk of the planet in the form of liquid metallic hydrogen. This exotic form of the most common of elements is possible only at enormous pressures, which only exist because of Jupiter's size. The outermost layer is composed primarily of ordinary hydrogen and helium. The atmosphere we see is just the very top of this deep layer. Water, carbon dioxide, methane and other simple molecules are also present in tiny amounts.
Jupiter is just about as large in diameter as a gas planet can be. If more material were to be added, it would be compressed by gravity such that the overall radius would increase only slightly. Stars can be larger only because the tremendous energy released in their cores pushes outward. Jupiter would have to aquire at least 80 times more material than it currently has to become a star.
Jupiter has rings like Saturn's, but much fainter and smaller. They were completely unexpected and only discovered when two of the Voyager 1 scientists insisted that they should at least check to see if any rings might be present. Unlike Saturn's, Jupiter's rings are dark.
When it is in the nighttime sky, Jupiter is often the brightest "star" in the sky (it is second only to Venus, which is seldom visible in a dark sky). The four Galilean moons are easily visible with binoculars; a few bands and the Great Red Spot can be seen with a small astronomical telescope. There's not much of a chance that life exists on Jupiter. Jupiterís interior is an environment of pressures up to three million times the sea-level pressure on earth, and temperatures as high as 10,000 degrees. Itíd be really amazing indeed if any life as we know it, could exist there.