Web Site Certificates
Sometimes, when visiting a web site, you may have seen a message stating that there is a problem with the site's certificate. If a web site has a valid certificate, it means that a certifying authority has verified that the web address actually belongs to that organization. In this way, certificates can protect you from "imposter" web sites that are trying to get your personal information. Usually you'll have the option of examining the certificate, accepting it forever, accepting it only for that particular visit, or choosing not to accept it. This article from US-CERT will tell you how to check the certificate's authenticity and help you decide what to do.