A "hijacker" is a type of spyware designed by certain online publishers that allow them to snoop on your browsing activity, invade your privacy, and flood you with those annoying pop-up ads.
A few of the symptoms are:
* Changing your "search" page to THEIR "pay-per-click" search page
* Changing your default homepage to another page (often porn sites)
* Transmitting Web sites you have viewed to their company servers