Spyware is basically any software which do spy work on your computer. Spyware software are bundled as a hidden program inside the free software or shareware applications that we download from the Internet.

Earlier Spyware used to be just an add on to adware programs to help the promoter of the free software to show targetted ads related to your browsing habits. Spyware collects information about your computer and your Internet activity, then sends it to the spyware creator database. Based on the information collected, spyware pop-ups ads on your computer.

Nowadays spyware do more than that, Spyware shows fake alerts and warning that your computer is infected with Trojans and Viruses. And then spyware pop-ups ads about fake antivirus software to remove these Trojan and Virus infections, which are all fake. When user click on this ads, trojans are downloaded and installed on users computer. Most common  fake anti virus programs which downloads trojans to your computer includes Advanced Antivirus, Antivirus 2009, Antivirus XP 2008.                        

Different types of spyware can do different things and that includes, showing unwanted or intrusive advertising, collecting private data and transmits it to a remote computer, steals usernames and passwords, steals online financial accounts, browser hijackers changing web browser settings like homepage, wrong search page results, webpage redirects. These changes can allow a hacker to gain control of the user’s computer and perform illegal actions, some of which can cause serious problems.

Spyware's also download and installs unwanted software files without user's knowledge or permission. Some Spyware even disables Antivirus software and Firewalls, and block access to security websites. Spyware can terminate a program or process, or even cause system errors which shut downs the computer. Some Spyware threats can collect e-mail addresses stored on a user’s computer and mass e-mail dangerous files which contains Trojans.