Spyware is computer software that is installed surreptitiously on a personal computer to intercept or take partial control over the user's interaction with the computer, without the user's informed consent.

Adware or advertising-supported software is any software package which automatically plays, displays, or downloads advertisements to a computer after the software is installed on it or while the application is being used. Some types of adware are also spyware and can be classified as privacy-invasive software.

Trojan horse, also known as a trojan, is malware that appears to perform a desirable function but in fact performs undisclosed malicious functions. Therefore, a computer worm or virus may be a Trojan horse. The term is derived from the classical story of the Trojan Horse.