Spyware is the name given to software that collects computer users' personal information without the users' willing or knowledge. Other terminology such as "malware" and "adware" which have a general and different meaning may also be used instead of spyware.

Spyware can perform various actions, such as keeping track of the keyboard when typing, keeping a record of the visited web sites, scanning the data on the hard disk, tracking the searches performed on the internet etc. which all mean a breach of secrecy of personal information. As a result, it can get hold of the passwords of e-mail, bank account etc. illegally, display pop-up windows and/or send spam e-mails for "personal advertisements" which are annoying, use up network and computer resources which causes the computer slow down, lag in opening up web pages, longer boot and action durations in general.

Spyware software are different from viruses, since they are not openly illegal and not designed for purposes that would directly pose a threat to the user, and install themselves onto the computer with the user’s consent or activities. Furthermore spyware do not copy themselves from one computer to another, like viruses do. However, the consent is obtained without any clarifying explanation on how these malicious software will work and perform on the computer.

Usually the user receives a notice asking to install an annex (add-on, extension, plug-in) on the web browser in order to open up or display the web page properly, and upon giving the authorization the spyware is installed. Also a web site that provides a free service (software, music, video, online games etc.) may impose the software to be installed on the computer. Similarly, some free software install spyware secretly or by trying to get the consent of the user while installing their software. Such sites or software developers sell and thus obtain a gain for having the software installed by selling the users’ personal or statistical information to people or groups who aim to advertise etc.