Elizabeth Proctor is the wife of John Proctor who had an affair with Abigail. While this sounds like an episode of your favorite soap opera, this is important to understand. Since Abigail and John broke up, she hasn't gotten over him; in fact she's still very much in love with him. So, when the opportunity arises, she calls Elizabeth a witch (by saying that Elizabeth sent her spirit out and stabbed her in the stomach with a needle) in the hopes that Elizabeth will be convicted and then executed. Once Elizabeth is gone, she hopes that she'll be able to live happily ever after with John.
Elizabeth and John are aware of her evil plan. In Act III, John tells Danforth
She [Abigail] thinks to dance on my wife's grave! And well she might, for I thought of her softly. God helped me I lusted, and there is a promise in such sweat.