It can be innuendo, verbal, written, physical or the display of offensive material. It does not have to occur repeatedly and ignorance is no defence. If it is reasonable in the circumstances that any sort of behaviour is unwelcome and offends, humiliates or intimidates it is sexual harassment. It is important to note that what is 'reasonable in the circumstances' can be perceived differently by the person doing the harassing or others in the workplace. However, any legal assessment of what is reasonable will depend on the facts of the case.
Sexual harassment has little to do with sex and a lot to do with power.
A form of sexual harassment is an act representing an abuse of power. Sexual harassment is often 'tolerated' by people because they feel that the harasser stands in a position of power in relation to themselves. Sexual Harassment is often male to female, but can also be male to male, female to female and female to male. Sexual Harassment can also be found when any unfavourable working conditions are directed at only one sex. This is called sex-based harassment.