Bilhah was a servant of Laban who he gave to his daughter Rachel to be her servant when Rachel married Jacob (see Jacob and Laban). Later, when Rachel found herself not having children (while her sister Leah, Jacob's other wife, was), Rachel arranged to have children through Bilhah (see Children of Jacob), thereby making Bilhah an ancestor of two of The Tribes Of Israel, Dan and Naphtali (Rachel later did have children of her own, Joseph and Benjamin).
Bilhah was Laban's maid who was given to his daughter Rachel when she became Jacob's wife:
"So Jacob served seven years for Rachel, and they seemed to him but a few days because of the love he had for her."
Then Jacob said to Laban, "Give me my wife that I may go in to her, for my time is completed."
"So Laban gathered together all the men of the place, and made a feast. But in the evening he took his daughter Leah and brought her to Jacob; and he went in to her. Laban gave his maid Zilpah to his daughter Leah to be her maid."
"And in the morning, behold, it was Leah; and Jacob said to Laban, "What is this you have done to me? Did I not serve with you for Rachel? Why then have you deceived me?"
"Laban said, "It is not so done in our country, to give the younger before the first-born. Complete the week of this one, and we will give you the other also in return for serving me another seven years."
"Jacob did so, and completed her week; then Laban gave him his daughter Rachel to wife. Laban gave his maid Bilhah to his daughter Rachel to be her maid. So Jacob went in to Rachel also, and he loved Rachel more than Leah, and served Laban for another seven years." (Genesis 29:20-30 RSV)
Bilhah became the mother of two of Jacob's sons, thereby making her the founding maternal ancestor of two of the tribes of Israel:
"When Rachel saw that she bore Jacob no children, she envied her sister; and she said to Jacob, "Give me children, or I shall die!"
"Jacob's anger was kindled against Rachel, and he said, "Am I in the place of God, who has withheld from you the fruit of the womb?"
"Then she said, "Here is my maid Bilhah; go in to her, that she may bear upon my knees, and even I may have children through her."
"So she gave him her maid Bilhah as a wife; and Jacob went in to her. And Bilhah conceived and bore Jacob a son. Then Rachel said, "God has judged me, and has also heard my voice and given me a son"; therefore she called his name Dan."
"Rachel's maid Bilhah conceived again and bore Jacob a second son. Then Rachel said, "With mighty wrestlings I have wrestled with my sister, and have prevailed"; so she called his name Naphtali." (Genesis 30:1-8 RSV)
Fact Finder: What incident with Bilhah caused Jacob's firstborn son, Reuben, to lose his rights of the firstborn to the sons of Joseph (i.e. Ephraim and Manasseh)?
Genesis 35:22 and 49:3-4, 1 Chronicles 5:1