Jupiter is the fifth planet of the solar system.

In ancient Roman mythology, Jupiter was the King of the Gods.
Jupiter is the largest planet in the solar system (diameter= 142,000 km = 88250 miles). And the planet that rotates around it's own axis the fastest of all. (once every 9 hours and 50 minutes). Jupiter is about 90% hydrogen, and 10% Helium. There is also some methane, water and ammonia.
Jupiter is the fourth brightest object in the sky (after the Sun, the Moon and Venus; at some times Mars is also brighter). It has been known since prehistoric times.
In 1610, Galileo discovered Jupiter's 4 largest moons (Io, Europa, Ganymede, Callisto)

The first spacecraft to visit Jupiter was pioneer 10 (1973).
Currently the spacecraft Galileo is orbiting the planet, and sending data for at least the next two years.

If Jupiter would have been 80 times bigger, it would probably have been a sun.
The temperature of the core lies somewhere around 20,000 K.
The temperature on Jupiter can drop to -150 degrees Celsius (-238 degrees Fahrenheit) 
The planet consists mostly of hydrogen, just as the sun and the 3 other big planets. Within the planet the hydrogen is liquid, on the surface it is frozen.

Jupiter has 16 satellites orbiting the planet.
These are (ordered by distance from planet):
Metis, Adrastea, Amalthea, Thebe, Io, Europa, Ganymede, Callisto, 
Leda, Himalia, Lysithea, Elara, Ananke, Carme, Pasiphae and Sinope.

Both Jupiter and the other gas planets have high velocity winds, confined in wide bands of latitude. The winds in adjecent bands go in opposite directions. The slight chemical and temperature difference give the bands different colors. Jupiters atmosphere is turbulent. Indicating that Jupiter's winds are driven by it's internal heat in stead of by solar heat.

The colors of the clouds correlate with their altitude, The lowest clouds are blue, the highest red.

Jupiter also has rings (Halo, main and Gossamer), but they are very small and faint. The Rings are dark, unlike Saturn's rings. They contain small rocks, but no ice.

The distance to the sun is 778.3 million km (483.7 million miles) and Jupiter rotates around the sun in 11.9 years. The planets' mass is 1.900e27 (318 times earth's mass and twice the mass of all the other planets in the solar system together)

Pictures of Jupiter (click to see the large picture):