Spyware is a type of malware that installs itself on a user's computer and collects data without the user's knowledge. The user is usually unaware of the presence of spyware on their computer and it can be difficult to detect.

Spyware programs usually collect the user's data from the Internet – the history of sites visited and logins and passwords to various services. Spyware is also capable of installing other programs or redirecting Internet browsers, again without the user's knowledge. Some spyware programs can even change various computer settings, slow down the Internet connection, or change the functions of other programs.

There are a large number of anti-spyware programs available to combat spyware, but some of these contain spyware themselves, masquerading as legitimate programs.