Phobos is the larger of Mars's two moons (though Deimos would disagree) and orbits about 9377 km from the center of Mars. It looks a bit unnatural, being nowhere near spherical and has a massive hole, which it claims to be a 'crater', but it is common knowledge that it is its over-seized belly-button.
Its name was originally thought to be named after the Greek god of the same name, meaning fear, but is now known to get it's name from the fact that soon after humans had first colonized Mars permanantly, they decided to send all of their tramps to one of Mars's moons. This was a very similar tactic to that used by the Brits when they sent all their scumbag criminals to Australia. This lead to the 'hobos' part of the name. The 'P' came from the fact that the whole planet is made of Phosphorus, of which the chemical symbol is P.
Role in Third Solar System War
In the Third Solar System War Phobos was one of the smaller bases of the Bad Guys. At this stage of the war they controlled Phobos, Deimos, Jupiter and all of its moons, Mercury, Venus and Saturn's rings. Phobos was mainly used in the production of Phosphorus bombs. As the whole moon was made of this element it was a very valuble base. However, towards the end of the war it was clear the Good Guys were going to be victorious. Their opponents realized that Phobos would soon be taken, and to prevent the use of the Phosphorus against them, the Bad Guys decided to take a chance with the old legend that the moon was hollow. They decided to try and blow the moon up by putting millions of bombs inside it. All citizens were evacuated and the explosives detonated. As it was hollow, Phobos exploded and all of the pieces crashed into Mars. Some see this as the turning point of the war, as one of these fragments collided with Mars' cache of nuclear bombs which they had been preparing to launch at Phobos and Deimos. This collision set off the nukes which promptly destroyed Mars, one of the Good Guys main bases.
Relationship between Phobos and Deimos
Phobos and Deimos are brothers, with Phobos being the older and the larger. However, Deimos frequently claims that he is the larger, even though everyone knows he is lying. He is just doing it for attention. They argue about everything, and Phobos normally wins. Mars, as their father (he reproduced a-sexually to avoid 'your ma' jokes) made their arguments into a competition. Whoever lost the argument would move gradually further away from him. This explains why Deimos is so far away from Mars. Deimos is frequently teased by his brother for his irregular shape (he looks like a nose. Fail.) but as Phobos is his father's favourite, he always gets away with it.