"12 Years a Slave" is a 2013 British American film directed by Steve McQueen and starring Chiwetel Ejiofor and Michael Fassbender. The film has received very positive reviews.
It is an adaptation of the homonymous memoir book written by Solomon Northup, so yes '12 Years a Slave' is based in a true story.
Solomon was born in New York and was a free black person until he was kidnapped and sold into slavery. He worked in plantations in Louisiana for 12 years until he was freed.
Solomon Northup's book was published in 1953 and sold 30.000 copies within 3 years.