A Browser Hijacker is a program, malicious or otherwise, that changes the home page or search settings of a web browser. There are some hijackers that will modify Windows shortcuts without your knowledge to automatically load a particular web site when you start them. This could cause web sites to launch in programs that normally would not browse the web such as Microsoft Word.
Browser Hijackers are typically bundled with free programs that you download off of the Internet. These programs are free because they include adware programs, including browser hijackers, which generate revenue for the developer when they are installed. Many hijackers can be removed by simply uninstalling them from the Add or Remove Programs or Uninstall Programs Windows control panel. Some are more tenacious and require anti-virus programs or specialized tools.