Spyware is software that is installed on a computing device without the end user's knowledge. Such software is controversial because even though it is sometimes installed for relatively innocuous reasons, it can violate the end user's privacy and has the potential to be abused.
When tracking software is abused, there can be a significant impact on privacy. For example, if a smartphone gets infected with mobile spyware that was sideloaded with a third party app, the phone's camera and microphone can be used to spy on nearby activity, record phone calls, log browsing activity and keystrokes, and monitor the phone owner's location.
Spyware can be difficult to detect; often, the first indication a user has that a computing device has been infected with spyware is a noticeable reduction in processor or network connection speeds and in the case of mobile devices -- data usage and battery life.
Antivirus software that includes antispyware protection should be used to find and remove spyware. To prevent spyware, users should only download software from trusted sources, read all disclosures when installing software, avoid clicking on pop-up ads and stay current with updates and patches for browser, operating system and application software.
Many internet users were first introduced to spyware in 1999, when a popular freeware game called "Elf Bowling" came bundled with tracking software.
This video from On Demand News explains how spyware works.