This is taken from an article I have on Biblical Polygyny but I do not have the authors name handy. Hope this helps.
Concubines
Some people today may think that concubinage in the Scriptures was a form of an immoral sexual relationship, similar to having a personal mistress. Nothing could be further from the truth. When the Hebrew word pilegesh (Strong's #6370) is used, it speaks of a female concubine that is MARRIED to her male partner.
Keturah is called Abraham's concubine in 1 Chronicles 1:32, but in Genesis 25:1, she is called Abraham's wife. David's ten concubines are indeed called concubines, but they are also called his wives by Yahweh Himself (2 Samuel 12:11; 16:21-22). In Judges 19 and 20, the Levite's concubine "committed whoring against him" (Judges 19:2) and left "her husband" (Judges 19:3). She is called a concubine in Judges 19:1,2,9,24,25,29; 20:4 and 5, yet at the same time, her male partner, the Levite, is called "her husband" in Judges 19:3 and 20:4. In addition, the concubines father is called the "father-in-law" (Judges 19:4,7,9), and the Levite is called the "son-in-law" (Judges 19:5). Clearly, concubinage is displayed as a marital commitment.
So then what is the difference between a "wife" and a "concubine"? Wives are free; concubines are not. It's as simple as that. Scripture portrays concubinage as the marriage of a slave girl, without the ceremony of betrothal and without property jointure.
'And when a man has intercourse with a woman who is a female servant, engaged to a man, and who has not at all been ransomed or redeemed, there should be an inquiry. But they are not put to death, because she was not free.' (Leviticus 19:20, The Scriptures)
In the above example, we have a slave girl who has been engaged or betrothed (married without the marriage having been consummated) to a man. Being a slave, she is not killed as a free woman would be for having committed adultery (Deuteronomy 22:23-24), precisely "because she was not free".
In Judges, the concubines husband is twice called "her master" (Judges 19:26,27). Other concubines are identified likewise. Bilhah, Jacob's concubine (Genesis 35:22), whom Rachel gave to him for a wife (Genesis 30:3-4), was a slave (Genesis 35:25, "female servant"). Likewise, Zilpah was a slave-wife (Genesis 35:26; 30:9). Marrying a slave girl was not only practiced, it was legislated in the Torah as well.
"And when a man sells his daughter to be a female servant, she does not go out as the male servants do. If she is displeasing in the eyes of her master who has engaged her to himself, then he shall let her be ransomed. He shall have no authority to sell her to a foreign people, because of him deceiving her. And if he has engaged her to his son, he is to do to her as is the right of daughters. If he takes another wife, her food, her covering, and her marriage rights are not to be diminished. And if he does not do these three for her, then she shall go out for naught, without silver." (Exodus 21:7-11, The Scriptures)
Notice that it doesn't say, "He cannot take another wife". It says, "If he takes another wife". Here we have another law concerning polygyny and it is not forbidden. Although there are people today who view concubinage as an evil deed, Leah, in the Scriptures, viewed it as part of that which pleased Elohim.
And Elohim listened to Le'ah, and she conceived and bore Ya'aqob (Jacob) a fifth son. And Le'ah said, "Elohim has given me my hire, because I have given my female servant to my husband." So she called his name Yissaskar. (Genesis 30:17-18, The Scriptures)
Leah had given Zilpah, her maidservant, to Jacob as a wife because she had perceived that she had stopped bearing children (Genesis 30:9). Yet, she continued to pray for more sons. Elohim heard her plea and Leah understood this to be a reward from Him for giving Jacob a concubine.