A browser hijacker (sometimes called hijackware) is a type of malware program that alters your computer's browser settings so that you are redirected to Web sites that you had no intention of visiting. Most browser hijackers alter default home pages and search pages to those of their customers, who pay for that service because of the traffic it generates. More virulent versions often: add bookmarks for pornographic Web sites to the users' own bookmark collection; generate pornographic pop-upadware and spyware, a browser hijacker may be installed as part of freeware installation. In this case, the browser hijacker is probably mentioned in the user agreement -- although, obviously, not identified as a browser hijacker. The problem is that users typically either ignore the fine print or only give it a cursory reading. A browser hijacker may also be installed without user permission, as the result of an infected e-mail, a file share, or a drive-by download. To avoid contamination, experts advise users to read user agreements carefully, and to be cautious about freeware downloads and e-mail messages from unknown sources.
Also see other forms of hijacking.