Spyware is placed on your computer in order to track your internet surfing habits. It knows every site you visit and every page on that site. Spyware also collects your personal information through software that tracks your actual keystrokes. If you fill in a form to make a purchase all your personal information, including name, address and credit card information can be tracked. The potential abuse of spyware tracking is also being discussed in some businesses that store secure information, such as credit card numbers or even medical records.
Adware is another type of spyware. It doesn't work by tracking your information as spyware does, but what it can do is actually change your browser settings without your consent. It can cause pop up ads to show on your computer. It can even place a new toolbar on your computer. Most people are unaware they even have any type of adware installed on their computer until their computer starts to slow down.
The spyware business is a billion dollar a year industry with people getting very rich selling the information they steal about you. There is a case currently in court in the State of New York against a spyware company. The outcome of this legal case will hopefully mean the end of secretly placed spyware.
In order to not fall prey to these types of malwares there are steps you can take. First, don't download freebies. A lot of services that are offered to you as "free" such as free music downloads are free per say, but the price you pay is allowing them to place spyware software or adware targeted advertisements on your computer. Read any agreements about the software very carefully before you download it.
Run an anti spyware software program often along with your virus protection software. Generally spyware and adware are designed to be difficult to remove from you computer and they leave behind "ticklers" which reinstall the software.