Janus was the god of beginnings and transitions in Roman mythology, and presided over passages, doors, gates and endings, as well as in transitional periods such as from war to peace. He was usually depicted as having two faces looking at opposite ways, one towards the past and the other towards the future. There was no equivalent of Janus in Greek mythology.
As a god of beginnings and transitions both in literal and abstract ways, he was also responsible for motion, changes, and time. He was present in the beginning of the world, guarding the gates of Heaven, and he also presided over the creation of religion, life, and even the gods. He was probably considered the most important Roman god, and his name was the first to be mentioned in prayers, regardless of which god the worshipper wanted to pray to.
In one of the myths in which Janus played an important role, Romulus, one of the founders of Rome, kidnapped the Sabine women, helped by his men. Janus saved the women by creating a volcanic hot spring which erupted and buried the kidnappers in the mixture of boiling water and volcanic ash.
See Also: Romulus