Manasseh was the son of Joseph and Asnat (who was given to Joseph as a gift from the Pharaoh) and brother of Ephraim.
Jacob adopts both Manasseh and his brother Ephraim as part of the tribe of Simeon and Reuben. Although Manasseh was technically the eldest son, he does not receive the greater blessing. Ephraim does, as Jacob foresaw that his descendants were more worthy of the blessing than Manasseh's (Genesis 48:13-20).
The tribe of Manasseh settled across the largest territory in Israel, stretching from the Jordan River across to the Mediterranean Sea. Half of the tribe settled east of the Jordan River but agreed to join the rest of the people in the war against the Philistines.
Sources: Bridger, David. Ed. The New Jewish Encyclopedia, NY: Behrman House, 1976; Navigating the Bible II