Spyware is known to be software, or sometimes hardware, that is installed in a computer outside of the user's knowledge so the spyware can gather information about the users and send it back to the spyware controller. Spyware is generally broken into two categories--surveillance spyware and advertising spyware.

The surveillance form of spyware includes keyloggers, screen capture devices and Trojans all used by companies, private detectives, law enforcement, intelligence agencies and suspicious spouses, among others. Advertising spyware are programs installed along with other software downloads due to activex controls. Many users are aware that adware is being installed when the download certain programs. This advertising spyware records information on the user's internet activities, passwords, email addresses, browsing history, buying habits and the computer's hardware and software configuration. They then use the information to target users with product specific advertisements.

Advertising spyware uses up a computer's resources making the computer administrator front the costs for running the applications. It also utilizes the computer's bandwidth to connect to the internet and upload information gather by the spyware program and then to send the advertisements to the user. Without making users aware of its presence, advertising spyware can be classified as theft.