What is the nature of Proctor's relationship with Abigail?

It's established early on in the play that John Proctor had once had an extra-marital affair with Abigail Williams, his servant girl. As a devoutly religious man and loving husband, John always felt rather uneasy about the affair, and so he ended it. For good measure, he also fired Abigail. Though brief, the illicit liaison was intense, and so it's not surprising that Abigail is keen to rekindle the relationship. As for John, although he still harbors feelings for Abigail, he's determined not to go back to her. He tells her in no uncertain terms that they must never be intimate again.
But hell hath no fury like a woman scorned, and Abigail, angered and hurt at being dumped, vows revenge upon John and his family. She uses false accusations of witchcraft to get back at her former lover, proving herself hell-bent on destroying the man she once claimed to love.