Spyware is a program that is installed on a computer without full knowledge of end user. In that it may be installed silently thru trojan or virus infections or thru installation of free software downloaded from the internet.
Spyware affects browser/computer settings, track surfing habits and may post ads, pop-ups or alerts about needing to buy either real or bogus software to fix issues with computer. Spy-ware can slow internet speed and change home page or search engine defaults.
Related Terms: Malware, Riskware, Greyware, Adware