In History: Tituba was a slave who worked for the Parris family during the Salem Witch Trials of 1692. The book Tituba, Reluctant Witch of Salem suggests that she first fell under suspicion after baking a "witch cake," made from rye meal and a witchcraft victim's urine. When fed to the dog that was believed to be the witch's "familiar" — an animal companion that serves the witch it's bound to — the spell would be broken and the identity of the witch revealed. Tituba was also the first woman to "confess" to being a witch. An excerpt from her examination, also featured in the book, claims she said, "The devil came to me and bid me serve him," and that she also described specific details of the witch community in Salem. In the end, though, she was thought to have been pressured into confessing, even beaten.
On Salem: Tituba (Ashley Madekwe) is a full-fledged witch. In fact, she's the one who turns Mary Sibley from mortal to witch and sets the events of the show in motion. Tituba has great power, much of which we suspect she hasn't revealed yet.