Mars is the second closest planet to the Earth (Venus gets closer), and in many ways is the nearest to the Earth in conditions. It is also the obvious place for the first genuinely self sustaining Human colony off this planet.
Mars has about a tenth of the mass of the Earth; its gravity is a bit over a third of the Earth's.
The atmosphere of Mars is much thinner than that of the Earth, having a little less than a hundredth of the air pressure. The air of Mars contains about 95 percent Carbon Dioxide, and 2.7 percent Nitrogen. There are smaller amounts of several other gases including a small amount of free Oxygen and some water vapour and a little bit of Methane. There is also some Carbon Monoxide which is poisonous to us although not to all life, and several inert gasses like those on the Earth.
Mars is colder than the Earth. At the Martian poles, the temperature certainly drops to minus 143 degrees C (minus 225 degrees F). This compares with the coldest temperature ever recorded on Earth (At Vostok in the Australian Antarctic Territory) of minus 89 degrees C. The hottest the surface of Mars gets at the equator is around 27 degrees C (80 degrees F), but the air temperature is practically always below freezing point.
Orbit and Seasons
Mars has a similar axial tilt to the Earth and has seasons, but because the Martian year is nearly twice as long as the Earth's, the seasons are longer. The Martian day is only a little bit longer than the Earth's day.
Mars does not have a strong magnetic field like the Earth's although there is evidence that in the past it had one. This means that the surface of Mars has much less protection from some of the more damaging particles from the Sun.
Mars has less water than the Earth, but still has substantial amounts. However, the water is not liquid on the surface. We cannot rule out liquid water underground, and there are a number of things that suggest it is likely. The water is mainly in the form of ice although there is water vapour in the atmosphere as well.