Scholars tend to agree that Stoker's inspiration for writing Dracula was the real-life story of Vlad the Impaler, a notorious historical figure who did horrible things to his captured enemies.  There were also other books written prior to Stoker's book that may have helped influence him, including fables and stories from Transylvania.  Another interesting bit of information is that when Stoker was sick as a child, his mother told him horror stories to keep him entertained.

Check the links below for more information about Stoker.  Good luck!