Spyware, sometimes called adware, refers to small programs that secretly monitor the sites you visit on the web.
You can think of spyware as being similar to a computer virus, in that both are installed on your PC without your knowledge (usually by downloading a file via email or the web) and both run invisibly on your PC, causing undesired effects.
However, spyware is sometimes thought to be less malicious than a virus, as spyware generally does not seek to delete your files or use your PC as a means to infect other PCs.
Currently, the main purpose of spyware appears to be to monitor the sites you visit on the internet, and hence serve you targeted advertisements via your web browser. Apart from violating your privacy, the detrimental effects of spyware may include a slowdown in your PC's performance and unexpected disconnections from AOL and the internet.