In 1989, then-Chief Justice Brian Dixon of the Supreme Court of Canada, in a unanimous decision, said "sexual harassment in the workplace may be broadly defined as unwelcome conduct of a sexual nature that detrimentally affects the work environment or leads to adverse job-related consequences for the victims of the harassment. It is...an abuse of power... [that] attacks the dignity and self-respect of the victim both as an employee and as a human being." (emphasis added)
Today, the italicized part of that decision serves to define sexual harassment in Canada. To prove sexual harassment, a person needs to show he or she endured unwelcome sexual attention and that it had detrimental or negative consequences. While the consequences can be the classic, "You show me some sexual attention and I'll let you keep your job," they don't have to go that far. A person who endures provocative pictures on the wall, or sexual jokes told in the lunchroom is often a person being subjected to sexual harassment. This is often referred to as a tainted, poisoned or hostile work environment.
Since the courts have ruled that sexual harassment is a form of sex discrimination, treating women negatively while treating men positively can constitute sexual harassment. In other words, if snide or rude comments are made to women but not to men, and they negatively impact the women's workplace comfort level, this is sexual harassment. The reverse is also true if men are the predominant targets.
My advice is to think of sexual harassment as more than the textbook case of a lecherous male boss. Non-sexual negative comments towards one particular gender or a man harassing a man, or a woman harassing a woman all qualify. That said, it should come as no surprise that most sexual harassment still involves a man sexually harassing a woman.
1) Call it what it is - not as a threat, but so it's not diminished. You don't have to shout it from the roof tops, but if a man (for example) is clearly sexually harassing a woman at work, then call him aside quickly and tell him what he's doing, why it’s wrong, why it's sexual harassment and what will happen if the behaviour doesn't change.
2) Talk to the person affected - this person is sometimes left out. In our example in #1 above, talk to the woman and ask her about the situation. She may have more to add than what you've just observed. Let her know what you're going to do and be sure to follow up when you've talked to the man who has harassed her.
3) Document, but don't become a lawyer - leave that to the lawyers. I find more people are worried about their paper work than they are about putting a stop to harassment. Any documentation will help if things go too far, but initially, if you can spend enough time to change the behaviour, you won't have to worry about anything going more formal. It's a bit of a balance.