God Mars was the Roman god of war, and was later identified with the Greek god Ares. The legends about him are mainly a transposition of the Greek ones, that's why he is considered son of Juno and Jupiter.
There is also a strange version about his birth, narrated by the Roman poet Ovidius: according to him, Juno was jealous because Jupiter's daughter Minerva was born without the help of a woman, so she wanted to have a child without the help of the male element. Flora, the goddess of vegetation, gave her a magic flower, and by simply touching it, Juno got pregnant with god Mars.
The month of March was dedicated to (and named after) Mars, that's why he was also considered the god of spring - but the beginning of spring also meant the beginning of the war campaigns (so much for "spring, season of love"). He was also celebrated in October, the month when the war campaign was over, and the weapons were purified and stored.
For his love-affair with Venus (the Roman counterpart of Aphrodite), please see the page on Ares. Among his many other adventures, the one with Anna Perenna was really funny. He asked old Anna to help him conquer Minerva, but the goddess wouldn't give up her vow of chastity. That's why Anna told god Mars that Minerva accepted, and that she would be waiting for him that night. But when Mars got into the room, in the darkness, guess who was there instead of Minerva? That's right, it was Anna, who lit a torch and laughed at him.
The Roman god Mars was considered the father of Romulus and Remus, the founders of the city of Rome. He fell in love with a vestal, Rhea Silvia, and seduced her. The twins were abandoned in the forest by their grandfather (who obviously wasn't very happy about this birth) and Mars sent a she-wolf to take care of them (the wolf and the woodpecker were his sacred animals). That's why the Romans considered themselves god Mars' sons - and so did other Italic tribes, too (as you can see, he was quite popular as a forefather).