can be absolutely infuriating. A browser hijack is when spyware takes over your internet settings, perhaps redirecting your internet searches and resetting your default home page. No matter what you do, you can't restore your homepage, and you will notice that your browser runs slower and slower. Hijackers may steer your browser via sites that you don't see, but which capture your personal information, so you can be spammed later on. Any software that resets your browser's settings to point to other sites is a hijacker.
How do you know you have a hijacked browser? You'll suddenly start seeing pages that you didn't request, and no matter how you try you can't stop your browser seemingly going wherever it pleases - often to porn sites. After a few days of this people have been known to lose the will to live.