A planet is a spherical object which revolves around, or orbits, a star.  (A moon is an object that revolves around a planet.)

There are nine major known planets within our own Solar System.  The planets Mercury, Venus, Earth, and Mars are known as the inner planets due to their proximity to the sun and are smaller that the others

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(with the exception of Pluto, the outermost planet).  The inner planets are primarily made of  rock and iron. 

The outer planets are Jupiter, Saturn, Uranus, Neptune and Pluto.  These are made primarily of hydrogen, helium, and ice.  

The planets widely vary in their atmosphere, temperature and other features.  Jupiter, which has many moons, is similar to a planetary system by itself.  One of its moons has volcanoes that create one of the hottest surfaces in the solar system.  Pluto, the most recent planet discovered, is also the farthest from the sun and is so cold that methane freezes on its surface.  Earth is the only planet in our solar system to support large amounts of water and life.