Programs that log all your key strokes. Not only can a keylogger get your passwords, it can record all information you type.

A good example would be when you are using an IM (instant messenger). Whatever you are typing to the other person is recorded. It is also possible for a keylogger to record what you write in an e-mail. It can also collect information about your bank account when using online banking.

A lot of your pop up ads may come from this type of spyware. Referred to as a Web Bug, this can cause pop up ads even when you are not online.

Your browsing habits can be tracked and this allows companies to send you ads relating to what you are browsing for. This would result in spam or other pop up ads.

Some people say they don't mind the pop ups, tracking or spam. But many of these people don't realize that this may be the only reason their computer is running slow. A good example is if you have a home network and one computer on that network is infected with spyware the result could be the entire network slowing down.

Programs that can open back doors into a system or network. A Trojan can disguise itself as a harmless warning on your computer or a harmless program.

An example would be an ALERT screen. The screen would say something like, Urgent warning, scan your computer now for problems. You click on it and a program is downloaded onto your computer. These programs will distribute, copy, or destroy the data on your computer. Think of a Trojan as something it's not. It appears to be something safe or innocent when it is actually harmful.