This is the definition I always understood of extortion:

Extortion is a crime in which one person forces another person to do something against his will, generally to give up money or other property, by threat of violence, property damage, damage to the person’s reputation, or extreme financial hardship. Extortion involves the victim’s consent to the crime, but that consent is obtained illegally.

Basically a judge will see it as extortion if you are threatening something criminal against a person. It's not really extortion to threaten that you'll do something you are legally entitled to do, however in some cases, your intent may play a role. For instance, there may not be a law against revealing secrets about someone, but there may be a law against doing so with malicious intent.