This comprehensive article from Chabad.org provides a survey of the Jewish understanding of angels and the role they play within Jewish theology. Shalom Aleichem, traditionally sung on Friday night before Kiddush, is addressed to the ministering angels that accompany a Jew home from synagogue services on Shabbat evening. The author, Rabbi Baruch Davidson, mentions some of the many Biblical references to angels, as well as who they are, what they do, and how they differ from humans.