How do security certificates work?

Security certificates work by electronically associating an identity (for example, a person sending an e-mail message) with a particular message, file, or website. This helps ensure, for example, that a message can only be opened by the person to whom it was sent. The certificate provides identity authentication that is verified by a certification authority, for the message, file, or website.
When you connect to a secure website, the site sends its certificate to Internet Explorer. Internet Explorer examines the certificate to help ensure that it was issued by a certification authority and has not expired.
Certificates can also be used to encrypt (scramble) a message or file. This helps to make sure that only the person to whom the message is sent can read it and to verify the identity of the person who sent the message to the person who receives it.