A planet is a celestial body orbiting a star.  In the solar system, where the Sun is the only star, there are eight major planets, with massive structure, and a number of dwarf planets that now include pluto.
 
The eight planets around the sun, in order of their proximity to the star, are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.  Pluto is no longer considered a planet and is classified as one of the three dwarf planets or plutoid, that also include Ceres and Eris.

Six of these planets, including the Earth, have natural satellites orbitting them.  The moon revolves around the Earth.

Jupiter is the largest planet with a diameter of 88,729 miles or 142,796 kilometers, while Mercury is the smallest with a diameter of only 3,031 miles or 4,878 kilometers.  The Earth has a diameter of 7,926 miles or 12,756 kilometers.  Pluto has a diameter of only 1,413 miles or 2,274 kilometers.

Mercury is closest to the Sun while Neptune is the farthest. Beyond Neptune is Pluto, which is the largest among the dwarf planets or plutoids.

Mercury is about 36 million miles or 57.9 kilometers from the Sun while the Earth is 93 million miles or 149.6 kilometers from the Sun.

The Earth is the most dense planet while Saturn is the least dense.  Jupiter is the most massive planet and has the strongest gravitational pull.

Aside from the major planets and plutoids, there are also asteroids, or minor planets made of rock and metal, that orbit the sun.