In short, not much. When you consider the definition of harassment—enduring the behavior feels like a condition of employment OR the conduct is severe or pervasive enough that a reasonable person would consider it intimidating, hostile or abusive—this describes workplace bullying too. If someone harasses a variety of protected characteristics (e.g., equal-opportunity harassment) that behavior is legal. Only when that harassment is pointed at a group based on their race, religion, sexual orientation, disability, or other characteristic, does it become unlawful behavior, called harassment.