The Solar System's four giant planets, against the Sun (to scale)
A "giant planet" is, as the name suggests, any massive planet. They are usually primarily composed of low-boiling-point materials (gases or ices), rather than rock or other solid matter, but massive solid planets are known to exist. There are four known giant planets that exist in the Solar System: Jupiter, Saturn, Uranus, and Neptune. Giant planets do not solely exist in the Solar System, many extrasolar giant planets known to be orbiting other stars.
Giant planets are sometimes referred to as a "jovian planet", in reference to the planet Jupiter. They are also sometimes known as gas giants. However, many astronomers apply the latter term only to Jupiter and Saturn, classifying Uranus and Neptune, which have different compositions, as "ice giants". Both names are potentially misleading: all of the giant planets consist primarily of fluids above their critical points, where distinct gas and liquid phases do not exist. The principal components are hydrogen and helium in the case of Jupiter and Saturn, and water, ammonia and methane in the case of Uranus and Neptune.
The defining differences between a very low-mass brown dwarf and a gas giant (~13 Jupiter mass) are debated. One school of thought is based on formation; the other, on the physics of the interior. Part of the debate concerns whether "brown dwarfs" must, by definition, have experienced nuclear fusion at some point in their history.