Jupiter is the largest planet in our system in fact it is more than twice as massive as all the other planets combined. The planet is made up of roughly 75% Hydrogen (by mass) and 25% Helium. There are trace amounts of methane, water, and heavier materials. This is very close to what the composition of the primordial nebula is believed to have been.
Jupiter is a gas planet where the gasses that make up the planet just get denser at greater depths until under the enormous pressure hydrogen takes the form of a metallic liquid. In this form the Hydrogen consists of a liquid of ionised protons and electrons. This liquid makes up most of Jupiter's mass. Inside this there is probably a rocky core which may be up to 15 Earth masses. (If this sounds large, note that the planet as a whole is 318 Earth masses). Since Hydrogen does not change phase suddenly the boundaries between the layers are probably not clearly defined.
Jupiter radiates more energy than it receives from the Sun and it's core is probably about 20,000 K. The heat is generated by the slow gravitational compression of the planet, not by nuclear fusion. Large as it is Jupiter is far too small (80 times) to sufficiently compress and heat its interior to initiate nuclear reactions. The interior heat probably causes convections within Jupiter's liquid layers and hence motions we see in the clouds. winds of 400 mph blow in opposite directions along adjacent bands of latitude.
Jupiter has a huge magnetic field that extends past the orbit of Saturn. Jupiter, like Saturn, has rings but much fainter and smaller. They were only discovered by the Voyager 1 spacecraft. Jupiter has four large Galilean moons and 12 small ones.