Many behaviors could be interpreted as sexually harassing, depending on the circumstances. Whether behavior is offensive or not depends on how it is perceived, not how it was intended. Sexual conduct is unwelcome whenever the person subjected to it considers it unwelcome.
Watch out for these "Danger Zones" :
- Inappropriate e-mail and sexually explicit internet use
- Comments on personal appearance
- Hugging, massages and other touching
- Unwanted, repeated requests for dates
- Off-color jokes and innuendo
- Sexually explicit profanity
- Nicknames and stereotypes
- Invading someone’s personal space
- Intrusive personal or sexually explicit questions
- Suggestive looks, gestures or sounds
- Ridiculing someone’s sexual orientation
- Spreading rumors about someone's sexual conduct
- Public displays of sexually explicit pictures, calendars, cartoons, etc.
This is a very basic list. You'll find definitions of terms related to sexual harassment and a comprehensive list of potentially problematic behaviors on this UN website.