Mars from space
Mars is a lot like Earth in some ways. Mars is not too far from the Sun to be solid (as Jupiter is), and it's not so close to the Sun that it is very hot (like Mercury). Like Earth, Mars rotates so that it has regular short days and nights, seasons, and a year more or less the same length as our year. But Mars is only about half as big as Earth - about 1000 miles (1500 kilometers) across, so Mars has less gravity than Earth does.
The atmosphere - the air - on Mars is thinner than on Earth, because there is less gravity. The air is mostly carbon dioxide and hardly any oxygen. This probably means that there are no plants on Mars that can photosynthesize using sunlight.
Probably there is no liquid water on Mars now, because the temperature is usually below freezing there, and the air is too thin. The air pressure is so low that water would just evaporate. There is ice made out of water though.
Bibliography and further reading about planets: