Mars is the fourth planet in the Sol system, positioned between Earth and the asteroid belt. Its senshi is Sailor Mars, and its two satellites are Phoboes and Deimos, who also have senshi.
Mars is often called The Red Planet due to it's uninhabited or unfoliaged state, and is often considered a sister planet to Earth. It has been the object of lore throughout Earth's cultures, and has been featured in myths, books, and movies (The Red Planet, Mission to Mars, etc.).
In Roman mythology, Mars is the god of war, and is represented by a spear and shield. Sailor Mars is known to have a fiery personality, her color is red, and her attacks use fire and weaponry images (i.e., Flame Sniper).
In the realm of Suburban Senshi, Mars was populated during the Silver Millennium, and was ruled by the House of Mars, of which Hino Rei was a princess in her previous life.
More about Mars can be found on its wikipedia page.