The inside of Jupiter. The solid core
is shown in brown
A gas giant is a large planet that has a solid core, but a very thick atmosphere. This means that most of the planet is made up of gas. These planets are very large.
In our outer Solar System, there are four gas giants: Jupiter, Saturn, Uranus, and Neptune. However, Uranus and Neptune are better called as ice giants.
Jupiter and Saturn are similar because they are made up of mostly hydrogen and helium. They both contain massive rocky cores that are bigger than Earth. Neptune and Uranus are similar because they are mainly made up of water and rock (a mixture of one or several minerals).
Gas giants have also been found around other stars than the sun. Most of those giant extrasolar planets are "hot giants" orbiting close to their star.