Firstly, the Movie is based upon the memoir of the same name by Solomon Northup, which was first published in 1855 and whose historicity is generally accepted (the edition edited by Sue Eakins - who researched the work in great detail - contains a lot of additional supporting information and documentation).
Now the Movie follows the book fairly closely, certainly by Hollywood standards. But changes have been made - mainly for the usual reasons of narrative pacing, but there are one or two which maybe don't sit quite right: The killing on the boat springs to mind. It's not in the book and is, to my mind, unnecessary. Also, in the book the plantation owners, Ford and Epps (not to mention Epps' wife) are much more rounded and complex personalities than they are presented as in the film.
hi! you'll find a few previous discussion in the FAQ (link on sidebar):