A smear campaign is the preemptive or retaliatory rewriting of your reputation — you're painted as unstable, abusive, or crazy to friends, family, coworkers, even courts, usually right before or after you leave. Its goals: isolate you, discredit anything you might reveal, and win the audience. Fighting it point-by-point usually feeds it.
They're calm and rehearsed; you're devastated and reactive — exactly the contrast the story needs. Every frantic correction you send becomes exhibit A. Smears also recruit helpers (flying monkeys) who apply social pressure sincerely.