What is a hijacker ? - Security Term

A hijacker is an attempt from a third party to take control of your web browser and use it for their own evil purposes. Some browser hijacks can be done to spread awareness of a particular website, but aren’t actually dangerous. In some cases, though, they can be malicious, and they can steal information like passwords you have saved into your browser automatically. Perhaps the most common type of browser hijack is done by a program that installs itself on your computer without you knowing. The program will add several favorites to your list without your knowledge, and it will change the start page of your browser and even some registry keys. When you attempt to reset your browser, you will be unable to do so.