To give the simplest answer, Earth is the third planet from the sun and Mars is the fourth. But there's lots of other differences. Earth has one Moon, Mars has two (Phobos and Deimos). Mars is slightly smaller than Earth and lacks an atmosphere, which makes it much colder when not in direct sunlight and much hotter when in direct sunlight. Also, Mars has no liquid fresh water on the surface. Scientists have detected some water on Mars' surface, but it is locked up in the poles. In addition, they think that Mars did have liquid fresh water on the surface at one time, but it has none now.