Spyware
Spyware is any application that collects information about your computer activities and then sends that information to another individual or company without your knowledge or permission. Spyware arrives bundled with freeware or shareware, through e-mail or instant messenger, as an ActiveX installation, or by someone with access to your computer. Once on your drive, spyware secretly installs itself and goes to work. Unlike traditional personalization or session cookies, spyware is difficult to detect, and difficult (if not impossible) for the average user to remove.