In the article before this (Computer Running Slowly), I wrote about extra programs running in the background, slowing up your computer.
Sometimes, those programs are malicious. They may want to reroute your searches to their own site. They may want to spy on you. They may want to use your computer to send spam. They may pop up advertising. They may block access to certain sites. Or they may do other things you wouldn’t want, either.
Some of them even replace the advertising that you would have seen on a web site.
Why are they doing all of this? Money.
There is big money in Internet advertising, especially when it’s done the adware route. They want to show you ads with their affiliate links rather than the web site’s affiliate links.
So, they are doing badly to two people — you and the web site’s operator. Web sites make money from advertising, mostly from advertising where someone clicks on an ad and then buys a product. By replacing the web site’s ad, or the web site’s affiliate code, the adware slime can rob the web site operator of legitimate sales commissions.