Mars is the fourth planet from the Sun. Its orbit takes it from 128 million miles out to about 155 million miles over the course of its 687-day long year. The length of the day on Mars is 24 hours and 40 minutes, completing a rotation around its axis in just slightly more time than Earth. Mars has a thin atmosphere - only about 0.6% as dense as Earth's - that is dominated by Carbon Dioxide. During summer days, the surface of Mars can reach temperatures as high as 70F; during winter nights, temperatures can plunge to -243F. Its rocky, dusty surface is dominated by igneous rocks, erosion products, and some sedimentary rocks. Mars has polar ice caps made of frozen water, and it's covered by valleys, deltas, and glacier scars - in its past, Mars was a wetter planet than today. In the past few decades, we've uncovered evidence that water exists in deposits around the planet - sometimes in great abundance. There's even evidence of seasonal flows of briny, liquid water in some craters.
Overall, Mars is the most Earthlike planet in the Solar System. It's this similarity that makes it most likely to be humanity's next home as we venture into the cosmos.