Secure web sites identify themselves using cryptography. In a phishing attack, a thief tries to impersonate such a site by omitting the cryptographic identification, or using one that's confusingly similar.
The Petname Tool can help you keep track of these cryptographic identifiers by letting you attach reminder notes to them. Before exchanging sensitive information with a site, you type in a name that will help you remember the site. From then on, whenever your browser is securely connected to that site, your name for it will be displayed. After following a hyperlink, just check that the Petname Tool is displaying the expected name. If so, the Petname Tool has checked that the cryptographic identifiers are also the expected ones.