Abigail Williams is one of the main characters in "The Crucible" and she is Reverend Parris' niece. Her parents have been killed prior the the play's beginning. Abigail had been a housekeeper for the Proctors but she was dismissed following her adulterous affair with John Proctor. She is the primary instigator of the accusations of witchcraft. And while her cruelty deserves criticism, she is also a sympathetic character because her need for attention derives from the death of her parents and the lack of a loving adult (aside from John Proctor, which is an unhealthy relationship). Abigail is also at the mercy of a sexually repressed, fanatically religious society (as are all the characters).
Tituba is Reverend Parris' slave from Barbados. She had been with the girls, dancing in the woods and suspicions of her are raised because of the history of Voodoo in her native Barbados. It seems that she went along with the mischief in the woods because she wanted to appease Abigail and the others. When the accusations start and the hysteria sets in, Tituba regresses to those superstitious roots and goes somewhat mad by the time she goes to prison.
Betty is Reverend Parris' daughter and is also one of the accusers. When the play opens, Betty is in bed having fainted and/or emotionally and physically overcome by the events in the woods. Although it is clear some of the other girls, namely Abby, have faked physical possession and real illness, it is unclear whether or not Betty is truly ill as a result of the events which transpired in the woods. In either case, it is clear that Betty is very impressionable and easily overwhelmed/influenced.