The security certificate (SSL certificate):

• identifies the owner of a website
• secures the exchange of information (using a security protocol such as TLS)

When a user connects to a secure site, the user's browser retrieves the site's security certificate. It carries out a number of validation operations to ensure that the certificate is compliant and has been issued for the website's address. If the certificate is not valid, the website's identity cannot be guaranteed for any of a number of reasons (for example, the site has been hacked into or a fake site is trying to pass itself off as the original site).

Most browsers now clearly show when a website's security certificate is invalid.