The most notorious of all mal-ware is adware. Generally speaking, adware is any software application in which ads are delivered via banners, popups, toolbars, etc. The developers of these applications include additional code that delivers the ads, which can be viewed through pop-up windows or through a bar that appears on a computer screen. The most infamous adware products include WhenU, Gator (Claria) and eZula, but there are hundreds.
Adware is added to your computer generally without your knowledge when you download anything free over the Internet. This method of downloading is known as "stealth software" or "drive-by downloads". Your computer could have been infected by mal-ware during online activities such as downloading music, pictures, shareware programs like screensavers and animated cursors, popup ads, email attachments or by even visiting some websites that use Active X coding to insert parasites onto your computer. Pay particular attention to the fine print when downloading anything over the Internet. Some programs include brief information in the Terms and Conditions related to serving you targeted ads based on your shopping habits.
Adware records what you are typing and what sites you are visiting. Ads are then served based on your activity. In a perfect world, adware would be a very popular application and serve you ads that are targeted to what you are interested in. However, unscrupulous publishers push way too many ads and products that are not targeted to your needs. We have had reports of consumers getting 15-20 popup ads at once