What this means is certainly not clear especially considering court cases like the Coltran case. Harassment provides a good topic for the social evolution researchers and I'm certain some professor somewhere is dreaming up just such a syllabus. Even though the definition of harassment is vague and the legal implications are often more vague we can look at harassment in the workplace and come up with some general guidelines. Get legal advice if you are going to terminate an employee for harassment. Make certain you have a solid paper trail.
Prevention is the best cure.
Review your harassment policy in your personnel manual. Make certain you have the policy posted in a public area. Do not hire employees that give 'warning' signs in an interview. Take certain your harassment policy is up to date. Law and precedent change so make sure you are current.