The First Wife
Sarah (Sarai) was Abraham's wife when they left their hometown of Ur of the Chaldees for the land of Canaan (Genesis 11:29 - 31). Sarah had been, sadly, unable to bear children (verse 30). God, fifteen years after the couple leaves Ur, promises Abraham he will produce a legitimate heir in his old age (Genesis 15:1 - 4). The Lord did not specify, however, when this miraculous event would occur!
Enter Hagar
Sarah, one year after God promised a child to Abraham, becomes impatient. Taking matters into her own hands, she offers her Egyptian slave Hagar to her husband.
And Sarai (Sarah) said to Abram (Abraham), "Behold now, the Lord has kept me from bearing. I pray you, go in to my maidservant. It may be that I may obtain children by her." (Genesis 16:2, HBFV throughout).
Hagar, with Sarah's permission, becomes the patriarch's concubine and produces Ishmael. Concubines, tolerated in the Old Testament, were generally treated as inferior mates compared to a wife (see Genesis 16:5 - 6, Genesis 21:14 - 19). Their children, most of the time, did not share in their father's inheritance (Genesis 21:9 - 12).
The Other Wife
Sarah, Abraham's beloved and beautiful mate, dies at the age of 127 (Genesis 23:1). It is only after this sad event that the Bible records the patriarch, now at least 140 years old, taking on a second wife named Keturah!
Then again Abraham took a wife, and her name was Keturah. And she bore him Zimran, and Jokshan, and Medan, and Midian, and Ishbak, and Shuah . . .
And Abraham gave all that he had to Isaac. But to the sons of the concubines which Abraham had, Abraham gave gifts. And he sent them away from Isaac his son while he still lived, eastward to the east country. (Genesis 25:1 - 2, 5 - 6).
Keturah is only mentioned four times in the Bible (Genesis 25:1, 4, 1Chronicles 1:32 - 33). Her name, in Hebrew, means "perfumed" or "incense" (Strong's Concordance #H6989). Other than this, Scripture is silent regarding her family background, where she lived and many other details.
Relationship Status
It is interesting to note that while Genesis 25:1 calls Keturah Abraham's wife, verse 6 of the same chapter as well as 1Chronicles 1:32 seems to contradict this by labeling her as a concubine.
What is clear from the Bible is that Keturah, even though called Abraham's wife, was still treated as a concubine in regard to her offspring. Keturah's sons, just like Hagar's son Ishmael, were not considered full heirs of their father's possessions. They were only given gifts (e.g. cattle and other goods) and then sent far to the east away from Isaac. Josephus, in regard to where these sons were sent, states the following.
"Now, for all these sons and grandsons, Abraham contrived to settle them in colonies; and they took possession of Troglodytis, and the country of Arabia the Happy, as far as it reaches to the Red Sea." (Antiquities of the Jews, Book 1, Chapter 15).
Troublemaking Descendent
Abraham and Keturah's descendants, through their son Midian, would play an active and usually negative role in the early history of Israel.
It was through traveling Midianite merchants that Jacob's (Israel's) sons sold their youngest brother Joseph into slavery (Genesis 37:28). Zipporah, Moses' wife, was a Midianite (Exodus 2:15 - 21).
It was also the Midianites, after Israel left Egyptian bondage, who actively attempted to lead them astray by having them marry their pagan women. Their behavior against God's people earned them a war in 1405 B.C. where Moses, in his last battle, destroys them (Numbers 25, 31).
Many years later, around 1152 B.C., the Israelites, due to their sins, are allowed to be oppressed by the Midianites. God, seven years later, raises up Gideon to save his people for Midian and their allies (Judges 6).
Conclusion
Sarah was the first wife of Abraham. The patriarch, after being married for many years, takes Hagar as his concubine at the behest of Sarah. Keturah, although considered Abraham's second or other wife, nevertheless has her offspring treated like she was a concubine.