Jupiter is the fifth planet in the Solar System, counted from the Sun, and the largest planet in it. It is named after the chief Roman god (or Zeus as he was known to the Greeks). It is categorized as a "gas giant." This is a reference to the low density of the planet, not to its manners.
 Red Spot
One of Jupiter's more fascinating features is the Great Red Spot, a huge weather system in its atmosphere that resembles a terrestrial cyclone, except for its relatively consistent appearance and its persistence (it has been observed for more than a century).
 Comet impact
In 1994 (was it really that long ago?) the comet Shoemaker-Levy 9, after breaking into fragments, collided with Jupiter. This was the first observed example of extraterrestrial objects colliding. The resultant "dark spots" in the Jovian atmosphere ranged up to 12 000 km (approximately equal to the diameter of the earth).
 Jupiter ignition
"Jupiter ignition" refers to the notion that Jupiter can be turned into star, popular in some crank/conspiracy theorist circles. It probably originates with the fact that Jupiter and other gas giants are sometimes called "failed stars" in popular science. It was a plot point in Arthur Clarke's 2010: Odyssey Two, where the second Monolith starts multiplying and "ignites" Jupiter by increasing its density. The main problem is that Jupiter simply doesn't have enough mass to sustain a star-like nuclear fusion reaction. There are objects that stand on the boundary between gas giants and stars - brown dwarfs - but the lower boundary of their mass is about 13 times the mass of Jupiter, and, despite the fact that some of them can fuse deuterium or lithium, they are still "cool" related to the star of the main consequence.
The idea has different permutations and is usually dusted off every time some major event happens on Jupiter. One possible originator of the conspiracy twist is Richard Hoagland, who said back in 2003 that NASA crashing the Galileo spacecraft into the planet would "ignite" it into a star. His "theory" being that the tiny amount of plutonium-238, that is used to power the craft through radioactive decay, would be crushed in the exceptionally high pressure of the atmosphere. This would cause an explosion due to nuclear fusion and turn Jupiter into a new star. Because to him all stars need is hydrogen and fire to be stars, mass doesn't matter. It has never explained why anyone would want to do that, since we would all die from radiation or the Earth would be torn apart by tidal forces.