well, a hacker pretends to be someone to gain your trust and get information from you or to get you to do something...
Like the person can call you and say there is a problem with your computer, then tell you the way to give them access to it while making you think they are fixing it....They can use this to get passwords, credit card numbers, multiple things....don't give information unless you are positive the person is who he says he is. normally the real person won't call you...that can be one of the red flags...