Did you know that Facebook, Google, and other companies track you while traversing the web? They following you to other sites and spy on everything you do.
Have you ever had a popup window tell you that your computer has been infected with a virus? Or have you had an alert tell you that Flash needs to be updated?
Many websites pretend to be something they are not. They can look exactly like the real one, yet really be a fake one that is trying to get your username and password.