The definition of a planet is a celestial body whose core is not hot enough to maintain nuclear fusion (otherwise it is a star). If it is in orbit around another planet it is a satellite (or moon), not a planet. The planet you are most familiar with is Earth! A planet can have many features, such as Mars and Venus, whose landscapes are magnificent! It also has many types of gases.
A Habitable planet is a planet where the temperatures are right for liquid water to exist.