Ephraim was the younger son of Joseph and Asenath, daughter of Potiphera. Ephraim was the ancestor of the tribe of Ephraim. His grandfather Jacob adopted him and his brother Manasseh to be among his own sons as heirs.
Having come to a place of power, with a wife of the priestly line in Egypt, Joseph honored his son with a name honoring God's blessings upon him in a foreign land. The word 'Ephraim' is a dual form of the word 'ephrah' which is from perah, meaning "fruitful." With a second son, Joseph was assured of continued prosperity in the land.
Ephraim was born into the household of Joseph, the son-in-law to the priest at On. His father had been promoted to second-in-command in Egypt after coming up from slavery by the grace of God. This was in the years of plenty as Joseph was administering the collection of grain and lands in anticipation of a great famine that was to come soon.
Knowing nothing but luxury, young Ephraim and his brother would find themselves pampered by their grandfather Jacob when he came into Egypt with his extended family. While still young, he would be favored over his older brother when Jacob sat up in his bed to bless the sons of his favorite son Joseph. Acting as a spokesman for God, Jacob used his blessing as a prophecy: the two boys would be counted among the founders of the "twelve tribes" and Ephraim, upon whom he put his right hand, would be the greater of the two.
When Jacob blessed his two sons, placing his right hand on the head of the younger boy, Joseph was upset. When he tried to correct his father, Jacob explained that although Manasseh would become the ancestor of many people, his younger brother would be the ancestor of many nations!
True to the prophecy in the blessing, the tribe of Ephraim would far outshine that of Manasseh. In the first census of the young confederation of tribes under the rule of God's covenant, Ephraim would number 40,500 soldiers plus women and children. Among these would be Joshua, son of Nun, who would become the successor of Moses.
When it came time to divide up the land, the tribe of Manasseh had chosen to keep land east of the Jordan. Then, when Ephraim got land with the ten tribes west of the river, certain cities that were in the bounds of the half-tribe of Manasseh were given to Ephraim. Many years later, when the united kingdom was divided by civil war, the northern kingdom of "Israel" would become largely apostate. The prophets would often refer to the ten tribes as simply "Ephraim."