Jupiter is the fourth brightest object in the sky (after the Sun, the Moon and Venus and, at some times, Mars is also brighter). Jupiter is the fifth planet from the Sun and by far the largest. Jupiter is more than twice as massive as all the other planets combined. Galileo's discovery, in 1610, of Jupiter's four large moons (Io, Europa, Ganymede and Callisto now known as the Galilean moons) was the first discovery of a center of motion apparently not centered on the Earth. It was a major point in favor of Copernicus's heliocentric theory of planetary motions. Galileo's outspoken support of the Copernican theory resulted in trial and imprisonment at the hands of the Inquisition.
Jupiter is a "Gas Giant" and does not have a solid surface. Its gaseous material simply gets denser with depth. What we see when looking at Jupiter is the tops of clouds high in its atmosphere. Jupiter's weather systems are easily visible in the form of cloud bands complete with swirls and eddies. The "Great Red Spot" is a cyclone like storm that has persisted for over 300 years.
Jupiter is about 90% hydrogen and 10% helium with traces of methane, water, ammonia and heavier elements. This is very close to the composition of the primordial "Solar Nebula" from which the entire solar system was formed. Saturn has a similar composition, but Uranus and Neptune have much less hydrogen and helium.