Well, it's the war where Lucifer led a third of the angels in Heaven against God because, due to his pride, he refused to serve humanity. The other two thirds either fought on God's side or were completely nuetral. Well, Lucifer was defeated and he, as well as all the angels who stood by him, were banished from Heaven and sent to Hell, their sins transforming them into demons. Those who did not fight on either side were (according to Dante) sent to the first circle of Hell which houses those who have not necassarily sinned but were either born before Christianity or did not believe in God but still lived a good life. The angels who sided with God remained in Heaven.