A spyware is a program that monitors and gathers user information for different purposes. Spyware programs usually run in the background, with their activities transparent to most users. Many users inadvertently agree to installing spyware by accepting the End User License Agreement (EULA) on certain free software.

Many users consider spyware an invasive form of data gathering. Spyware may also cause a general degradation in both network connection and system performance.

The state of California classifies spyware as: programs that are installed under deceptive circumstances; software that hides in personal computers; software that secretly monitors user activity; keylogging software; and software that collects Web browsing histories.