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.