You can learn more about planets in the Solar System section but lets give a quick overview that a planet is made up of heavy elements. Stars are mainly made up of Hydrogen and Helium.
Planets generally have all of the other elements in their crust and core.
You will find large amounts of Iron and minerals. Aslo, planets do not have the nuclear reactions igniting their atmosphere. When you think about the characteristics that make a planet a planet, think about the Earth. We have an atmosphere that is not on fire. The Earth is basically a large rock.