Spyware is considered malware (malicious software) because it installs itself on your computer without your knowledge. Then it watches your computer habits, compromising your privacy.
Spyware is also annoying because it pushes unwanted advertising at you. Pop-up ads appear out of nowhere on your desktop. And because it is spying on you, it pushes ads at you that it thinks you'll click on.
If that's not enough, spyware can clog your system's memory and use space on your hard drive, causing performance slowdowns. It can get so cloggy that it'll make your computer unusable.
Some spyware programs can even capture your keystrokes and send them to a third party. This can potentially expose your user IDs and passwords to thieves .
It can also allow installation of an electronic back door that allows bad people to log into your computer remotely and use it for their own purposes, such as sending spam or launching malicious attacks on other computers on the Internet.
Spyware can change settings and hijack your web browser so a rogue homepage is loaded every time you surf the Web.
On top of all this, some spyware embeds itself so cleverly and deeply into your system that removing it requires a lot of computer expertise. You almost have to be as computer-savvy as a nine-year-old.