Sexual harrassment is a type of discrimination based on sex. It involves unwelcome comments or actions based on sex usually taking place more than once over a period of time.
An obvious example of sexual harrassment would be an employer demanding sexual favours in return for continuing employment. A less obvious example would be sexually related comments or jokes at work or school that make someone uncomfortable. An even less obvious example would be where someone is subjected to extra scrutiny at work or school, without any sexual overtones, merely because of their gender.
That last point is worth emphasizing. Sexual harrassment is not necessarily related to sexual activity. If a woman is doing what is seen as traditionally a job for a man and she is treated differently because she is a woman, that amounts to sexual harrassment. A male student doing what is traditionally seen as woman's course who is treated differently may be sexually harrassed.
It is also important to remember sexual harrassment is not limited to employment or school -- it can occur in the provision of services or otherwise. Also in the employment context, sexual harrassment is not always an employer; coworkers can sexually harrass each other although the employer has a duty to make sure that does not happen.