Little is known about her, as her origins are obscure and she died at a young age of milk sickness. According to Herndon, Lincoln believed her to be the product of an illegitimate union
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Milk sickness, also known as tremetol vomiting or, in animals, as trembles, is a kind of poisoning, characterized by trembling, vomiting, and severe intestinal pain, that affects individuals who ingest milk, other dairy products, or meat from a cow that has fed on white snakeroot plant, which contains the poison tremetol.

Although very rare today, milk sickness claimed thousands of lives among migrants to the Midwest in the early 19th century in the United States, especially in frontier areas along the Ohio River Valley and its tributaries where white snakeroot was prevalent. New settlers were unfamiliar with the plant and its properties. A notable victim was Nancy Hanks Lincoln, the mother of Abraham Lincoln, who died in 1818.[1] Nursing calves and lambs may have died from their mothers' milk contaminated with snakeroot, although the adult cows and sheep showed no signs of poisoning. Cattle, horses, and sheep are the animals most often poisoned.
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Nancy Hanks found in:
Marriage Index: IL, IN, KY, OH, TN 1720-1926
Spouse: Thomas Lincoln
Marriage Date: Jun 10, 1806
Location: Washington Co., Kentucky
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Nancy's birth date has long been attributed to February 5, 1784. However, in 'Lincoln Lore' Warren pointed out that the wedding bond of Nancy Hanks and Tom Lincoln was signed by "Richard Berry, gardin". Had Nancy that June tenth, 1806, being twenty-one, she would not have had a guardian, So she must have been born after June 10, 1785. As February 5ths were observed as her birthdays, Nancy was probably born 2-5-1786 or 2-5-1787.
Courtesy of the late David S. Keiser of Greensboro, North Carolina
How I Found Nancy Hanks' Father, Thomas Hanks, Without Trying
By David S. Keiser, 3-23-1970
http://www.geocities.ws/marianapolis99/keiser.htm
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There is much mystery concerning Nancy Hanks, the parentage of herself as well as her son Abraham Lincoln:
Copied at Abbeville Courthouse, South Carolina District of Abbeville:

This agreement made and entered into on the 19th day of February 1809. I John C. Calhoun of the said state and district, of the first part and Nancy Hanks of the second part, for and in the consideration of the sum of $100.00 per year, to be paid to Nancy Hanks, for the support of an illegitimate son born February 12, 1809. The said sum of money to be paid to Christopher Orr, who shall act as guardian for said child.

John C. Calhoun Signed in the presence of, and on the above mentioned date. Witness Christopher Orr, Robert Brown Norris, Thomas Lincoln.
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Some sources say that Nancy Hanks was already with child when she married Thomas Lincoln in 1806. In taking the above reference into consideration Nancy and Thomas Lincoln would have been married about 3 years when this 'illegitimate son (Abraham Lincoln) was born in 1809.