In the field of computing the term spyware refers to a broad category of malicious software designed to intercept or take partial control of a computer's operation without the informed consent of that machine's owner or legitimate user. While the term taken literally suggests software that surreptitiously monitors the user, it has come to refer more broadly to software that subverts the computer's operation for the benefit of a third party.

In simpler terms, spyware is a type of program that watches what users do on their computer and sends this information over the Internet.

Spyware can collect many different types of information about a user. More benign programs can attempt to track what types of websites a user visits and send this information to an advertisement agency. More malicious versions can try to  record what a user types to try to intercept passwords or credit card numbers. Yet other versions simply launch popups with advertisements.

 Spyware differs from viruses and worms in that it does not usually self-replicate. Like many recent viruses, however, spyware – by design – exploits infected computers for commercial gain. Typical tactics furthering this goal include delivery of unsolicited pop-up advertisements; theft of personal information (including financial information such as credit card numbers); monitoring of Web-browsing activity for marketing purposes; or routing of HTTP requests to advertising sites.

As of 2006, spyware has become one of the pre-eminent security threats to computer-systems running Microsoft Windows operating-systems (and especially to users of Internet Explorer because of that browser's collaboration with the Windows operating system). Some malware on the Linux and  Mac OS X platforms has behavior similar to Windows spyware, but to date has not become anywhere near as widespread. In an estimate based on customer sent scan logs, Webroot, makers of Spy Sweeper, said that 9 out of 10 computers connected to the internet are infected and 86% of those surveyed suffered a monetary loss due to spyware.

Spyware does not directly spread in the manner of a computer virus or worm: generally, an infected system does not attempt to transmit the infection to other computers. Instead, spyware gets on a system through deception of the user or through exploitation of software vulnerabilities.

The most direct route by which spyware can infect a computer involves the user installing it. However, users tend not to install software if they know that it will disrupt their working environment and compromise their privacy. So many spyware programs deceive the users, either by piggybacking on a piece of desirable software such as Kazaa, or by tricking the users to do something that installs the software without them realising. Recently, spyware has come to include "rogue anti-spyware" programs, which masquerade as security software while actually doing damage.

Classically, a Trojan horse, by definition, smuggles in something dangerous in the guise of something desirable. Some spyware programs get spread in just this manner. The distributor of spyware presents the program as a useful utility — for instance as a "Web accelerator" or as a helpful software agent. Users download and install the software without immediately suspecting that it could cause harm. For example, Bonzi Buddy, a spyware program targeted at children, claims that:

He will explore the Internet with you as your very own friend and sidekick! He can talk, walk, joke, browse, search, e-mail, and download like no other friend you've ever had! He even has the ability to compare prices on the products you love and help you save money! Best of all, he's FREE!