The traditional Catholic Prayer to Saint Michael the Archangel became an instituted prayer of the Catholic Church in 1886 under the papacy of Pope Leo XIII. There are many legends that surround the prayer's composition, but they have not been officially verified by the Catholic Church.
When Pope Leo instituted the Prayer to Saint Michael the Archangel, he decreed that it should be said after Mass (and it is still said after many Tridentine Low Masses as of 2015). Pope Paul VI did not include the prayer in his revisions of the Mass in 1968.
The basic theme of the prayer is that of invoking the protection of St. Michael the Archangel against the forces of evil. It specifically asks that the angel defend those still on earth as they do battle, and to help them avoid the snares laid for them by Satan. Invoking St. Michael as a leader of the host of angels, it closes by asking him to send Satan and the other malicious spirits back to hell from which they came.
Some traditions believe that St. Michael was the one to cast Lucifer (Satan) out of heaven after the latter's primordial rebellion against God. Art work often depicts him as a soldier bearing a sword.