In Roman mythology, Janus was the god of gates, choices, doors, doorways, beginnings and endings. He was sometimes treated as a minor deity. He is most often depicted as having two faces or heads, facing in opposite directions. Janus was well-respected and highly regarded as a god by the Romans, and so his dual-faced image could be found on many things ranging from city gates and Roman coins.
Janus presided over the beginning and ending of conflict, and hence war and peace. The doors of his temple were open in times of war, and closed to mark the peace. As a god of transitions, he had functions pertaining to birth and to journeys and exchange, and in his association with Portunus, a similar harbor and gateway god, he was concerned with traveling, trading and shipping.
He reigned over all dual matters, such as birth and death, as well as traveling, trading and shipping, making his duty overlap with the god Mercury. Janus was also invoked during every religious ceremony in the Roman Empire, even during those of Jupiter. His most prominent remnant in modern culture is his namesake, the month of January, which begins the new year.
There is no Greek equivalent to Janus, since the Romans claimed him distinctively as their own. In Roman times, his name would be spelt Ianvs, because the Romans did not have the letters J or U.