Popups are often used by many websites for useful purposes, for example clicking a link and displaying a larger copy of an image or a help menu. However, they are often used to display annoying adverts and to load malicious software onto your computer.

www.popupcheck.com is a great site that will allow you to test your browsers skills in blocking malicious popups and allowing good ones.

When blocking popups there are three areas to think about:
1) The grey 'messenger' popups are a special type, and have nothing to do with websites. See the messenger spammessenger spam page for more info.
2) If your system is infected with spyware, often webpages will get hijacked and redirected, or pages will just open out of the blue. See securitysecurity section for info on cleaning up your system.
3) If your browsers own blocking techniques aren't good enough, consider some extra software such as the Google Toolbarhttp://toolbar.google.com.

If you dont know what a popup is, click here for an example.