Mars is the fourth Planet in the Terran Solar System, and was naturally the first planet ever to be colonized by Humans after extreme terraforming, due to the fact that it's the closest habitable planet to Earth. As such, it probably holds a population close to Earth's. It has two moons, Phobos and Deimos.
Mars is slightly smaller than Earth and has probably been terraformed or has closed colonies to sustain its population. It is likely that Mars is an industrial planet full of shipyards, mining facilities, and food production factories. The planet is not featured in the Killzone games, only mentioned.
The food company, Planet Yummy, is headquartered on Mars. Its products can be seen in Vektan malls, so it is presumed that it's an important food provider for many planets. Planet Yummy has a commercial playing on the UNCB.