Janus is the Roman god of doorways, beginnings, change, and transition. He was a very important figure in the Roman pantheon, typically being named first during prayers, and the cult of Janus was at one point very widespread in Rome. Janus lives on in some surprising places; the first month of the year, January, is named for Janus, as are janitors, the keepers of doors and keys.
The most striking and memorable feature of Janus is probably his two or sometimes four faces. In classical art, he was depicted with two faces which pointed in opposite directions, illustrating his role as a guardian of doorways; with two faces, he could look out on either side of the doorway to see what was happening. On occasion, busts of Janus showed four faces instead of two, and in full statues, Janus typically holds a key in his right hand.
Janus' original role was probably as the guardian of doorways, with his later duties being added by subsequent worshipers. The link between doorways and new beginnings is pretty obvious, as is the association with change. Janus was often worshiped at major life events like coming of age parties, weddings, and new year's parties, with those present praying to Janus for good fortune.
Janus also came to be associated with war, thanks to the Janus Geminus, a large temple in Rome. By tradition, the doors of this temple were left open during periods of war, and closed in periods of peace. Soldiers often visited the temple to pass through its doors in the hopes of being blessed by the god so that they could perform well in war.
The god was also linked with endings, as a doorway can appear at the end of a journey just as it can at at beginnings. In some regions, Janus was said to be capable of predicting events, thanks to his double-faced head, which presumably didn't miss much. Romans sought advice from Janus about major decisions, hoping that he could illuminate the best path to follow.
In the modern era, Janus is also used as a metaphor for the often complex nature of humanity. Janus is often referenced in art with two-faced individuals who have markedly different faces, such as the face of a man and the face of a woman. The Romans probably did not think about Janus in this way, although they undoubtedly thought about duality and betrayal, just as modern people do.