The outer solar system is much further out than the inner solar system (5-40 AU compared to Mars' 1.5 AU). Thus it is much colder there, and icy planetesimals could form and survive. After hydrogen (and inert helium), the next most common elements are C,N,O. They make molecules with hydrogen: water (OH₂), ammonia (NH₃), and methane (CH₄) which will freeze in the outer solar system. Thus the planetesimals built up more massive planets; after about 15 Earth masses the planets began to attract H and He directly from the solar nebula (the disk of gas and dust around the Sun). So were born the gas giants (click links for images). Jupiter got the most, Saturn did OK, but Uranus and Neptune just began capturing gas when the solar nebula dissipated. So Jupiter is most like the Sun in composition (90% hydrogen), Saturn is similar, but Uranus and Neptune have a larger percentage of water, rock and ice (though lower than the terrestrial planets). All have low densities because of the dominance of icy planetesimals. None has a solid surface. Jupiter has a possible rocky core, a "mantle" of metallic hydrogen, and very thick atmosphere. Saturn is similar. Uranus and Neptune have likely rocky cores, possible water "mantles" and thick atmospheres.

The "surface" of all the gas giants is actually clouds of ammonia, methane (outer planets), and more complex organic compounds. These planets can hold onto all their gas due to their cold temperature and high surface gravity. All have shorter days than the Earth producing very high surface rotation velocities. These, combined with convective motions from the interior, give rise to a banded appearance (Uranus has less convection for unknown reasons, so no bands). Storms can appear on the planets (also due to rotation) and last for much longer than on Earth (the Red Spot on Jupiter is a "hurricane" hundreds of years old). Because Jupiter and Saturn have metallic hydrogen (electrically conductive), convection, and rapid rotation, they have strong magnetic dynamos. Jupiter's magnetosphere is the largest thing in the Solar System besides the Sun.

The outer planets each have multiple moon systems. Jupiter's Galilean moons are each different (and look as though they formed in a mini-disk around Jupiter, with higher densities closer to Jupiter). Io is rocky, and currently the most volcanically active body in the Solar System. This is due to internal heating of Io by tidal stresses caused by Jupiter and Europa. Europa is an icy moon that is also heated by tidal stresses, so we think there may be a thick liquid water ocean below its frozen exterior. These moons have "young" surfaces that show few craters, but most of the other moons in the outer solar system show signs of early intense cratering and giant impacts.

Each of the gas giants also has a ring system (though Saturn's is far more impressive than the others). The rings are composed of rock and ice particles from dust sized to giant boulders. They are subject to drifting, and so must all be relatively young (and require replenishment to remain). One source of ring material is collisions of moons, another could be the destruction of moons when they come within the Roche limit of the planet. This is the point within which tidal forces due to the planet can disrupt a body which is held together primarily by its own gravity (you can tell when a body is like that because it will assume a spherical shape). The rings have a banded structure with gaps; this is due to orbital resonances with the moons (when the orbital period of a ring particle is an integral fraction of the moon's orbital period). In some cases there are very small "shepard moons" orbiting within the rings which will also produce gaps or very thin rings.