How do security certificates work?
A security certificate, whether it is a personal certificate or a Web site certificate, associates an identity with a "public key." Only the owner of the certificate knows the corresponding "private key." The "private key" allows the owner to make a "digital signature" or decrypt information encrypted with the corresponding "public key." When you send your certificate to other people, you are actually giving them your public key, so they can send you encrypted information that only you can decrypt and read with your private key (personal certificate).
The digital signature component of a security certificate is your electronic identity card. The digital signature tells the recipient that the information actually came from you and has not been forged or tampered with.
Before you can start sending encrypted or digitally signed information, you must obtain a certificate and set up Internet Explorer to use it. When you visit a secure Web site (one whose address starts with "https"), the site automatically sends you its certificate.