Hagar was the handmaid of Abraham and the mother of Ishmael (Hebrew meaning: God will hear). Abraham has relations with Hagar because Sarah, his wife, could not have children (Genesis 7:2). But a short time later Sarah gives birth to Isaac, the ancestor of the Jewish people. Sarah feels that Ishmael's presence will hurt Isaac's rightful place as the father of a great nation and she commands Abraham to drive Hagar away (Genesis 21:10). God rescues both Hagar and her son from death and promises to make Ishmael a great nation (Genesis 21:12-18).
Sources: Bridger, David. Ed. The New Jewish Encyclopedia, NY: Behrman House, 1976; Navigating the Bible II