The Tribe of Ephraim is a half tribe of Joseph that was risen to the level of the 11 other tribes. And since Joseph received the birthright of a double portion, Ephraim (along with Manasseh) received a full portion from Jacob (Israel) who claimed the pair of grandsons as his own sons in order to bless them accordingly.

One thing to note about Ephraim is that they were the largest tribe of the Northen Kingdom of Israel. This is why there are times in books like Hosea, Isaiah and Psalms where "Ephraim" is used to describe the whole kingdom just as the other kingdom is known as Judah (which also included Benjaminites and some Levites).