Spyware can do any number of things once it's installed on your computer.
At a minimum, most spyware runs as an application in the background as soon as you start your computer up, hogging RAM and processor power. It can generate endless pop-up ads that make your Web browser so slow it becomes unusable. It can reset your browser's home page to display an ad every time you open it. Some spyware redirects your Web searches, controlling the results you see and making your search engine practically useless. It can also modify the dynamically linked libraries (DLLs) your computer uses to connect to the Internet, causing connectivity failures that are hard to diagnose. At its very worst, spyware can record the words you type, your Web browsing history, passwords and other private information.
Certain types of spyware can modify your Internet settings so that if you connect through dial-up service, your modem dials out to expensive, pay telephone numbers. Like a bad guest, some spyware changes your firewall settings, inviting in more unwanted pieces of software. There are even some forms that are smart enough to know when you try to remove them in the Windows registry and intercept your attempts to do so.
The point of all this from the spyware makers' perspective isn't always clear. One reason it's used is to pad advertisers' Web traffic statistics. If they can force your computer to show you tons of pop-up ads and fake search results, they can claim credit for displaying that ad to you over and over again. And each time you click the ad by accident, they can count that as someone expressing interest in the advertised product.
Another use of spyware is to steal affiliate credits. Major shopping sites like Amazon and eBay offer credit to a Web site that successfully directs traffic to their item pages. Certain spyware applications capture your requests to view sites like Amazon and eBay and then take the credit for sending you there.