A gas giant is a generic astronomical term for any large planet that is not composed mostly of rock or other solid matter. Gas giants may still have a solid core - in fact, it is expected that such a core is probably required for a gas giant to form - but the majority of its mass is in the form of gas (or gas compressed into a liquid state).
The term 'gas giant' is actually somewhat of a misnomer. For example, Jupiter has a thick atmosphere composed of mostly hydrogen gas and helium, with trace amounts of other chemicals such as ammonia. However, the majority of the planet's mass is liquid hydrogen, possibly with a rocky or nickel-iron core. The composition of the other gas giants is similar, though Uranus and Neptune have more water, ammonia, and methane. The lower layers of liquid hydrogen inside gas giants is often so highly compressed that it becomes metallic in nature; metallic hydrogen is stable only under such enormous pressures.
Many of the extrasolar planets which have been discovered so far appear to be gas giants, making it appear likely that this type of planet is very common throughout the Universe. However, it is important to note that the detection techniques that have been used to identify extrasolar planets so far (detecting doppler shift in the star's spectrum due to the wobble induced by the planet's orbit) are much more adept at detecting giant planets than smaller ones and therefore this sample may be biased.
The upper mass limit of a gas giant planet is approximately 80? times that of Jupiter (around 0.04 times the mass of the Sun). Above this point, the intense heat and pressure at the planet's core begins to induce nuclear fusion and the planet ignites to become a red dwarf star.
Gas giant planets that form directly from a collapsing nebula rather than accreting from a protostellar disk like other planets are more properly termed brown dwarf stars. Brown dwarfs do not have nuclear fusion ongoing in their cores (those that do are red dwarves instead), but can still glow red and infrared from the leftover heat generated by their formation. A few potential brown dwarf candidates have been detected, and they are thought to be the most common type of star in the galaxy.