Mars is the fourth planet of the solar system.
Of all planets of the solar system, mars resembles the earth the most. It has vulcano's, deserts, and craters. Mars' rotation around it's own axis takes 24 hours and 37 minutes, and the planet has an atmosphere, just dense enough to allow simple life forms. 
The planet also has poles and a large iron core. 
On Mars lies the highest mountain of our solar system, a vulcano more then 25 km high.
Mars is called the red planet, this is because of the big red clouds that sometimes hang above the surface.

The planet probably has a 1700 km core, a molten rocky mantle, a little denser then earth's and a thin crust. Mars has a relatively low density, which indicates that the core probably contains a lot of sulfer in addition to the iron.

Mars' orbit is elliptical, which results in a 30 degree C variation in temperature at the subsolar point. The average temperature on Mars is -55 C (-67 F). The surface temperature of the planet ranges from -133 C (-207 F) to 27 C (80 F).

Mars doesn't seem to have the tectonic plates which earth and Mercury do have. There is no evidence of surface motion, such as folded mountains. There is also no evidence of current vulcanic activity, but new data from the Mars global surveyor indicates that Mars had tectonic activity during it's early history.

There is very clear evidence of erosion on the planet, which indicates that Mars once had rivers, and maybe even oceans. This was about 4 billion years ago.
The planet has a very thin atmosphere, which mostly contains tiny amounts of remaining carbon-dioxide (95.3%), nitrogen (2.7%), argon (1.6%) and traces of oxygen (0.15%) and water (0.03%). 
The average pressure on Mars' surface is 7 millibar (less then 1% of earth's pressure). It is thick enough to support high velocity winds and big dust storms.
Mars' thin atmosphere produces a greenhouse effect, which is only strong enough to raise the temperature by 5 K

The first spacecraft to visit Mars was Mariner 4 in 1965. Several others followed including Mars 2, the first spacecraft to land on Mars and the two Viking landers in 1976. Ending a long 20 year hiatus, Mars Pathfinder landed successfully on Mars on 1997 July 4 

The Viking landers performed experiments to determine the existence of life on Mars. The results were somewhat ambiguous but most scientists now believe that they show no evidence for life on Mars (there is still some controversy, however). Optimists point out that only two tiny samples were measured and not from the most favorable locations. More experiments will be done by future missions to Mars.

Mars has two small satellites (Phobos and Deimos), which orbit very close to the planet.

Mars has a diameter of 6,794 km (4,223 miles)
The planet rotates around the sun in 687 days. The distance to the sun is 227.9 million km (141.6 million miles). The planet's mass is 6.2419e23 kg.

Pictures of Mars (click to see the large picture):