A CER file is a security file provided by a third party Certificate Authority, such as VeriSign or Thwate, that verifies the authenticity of a website. It is installed on a web server to establish the validity of a certain website hosted on the server.
CER files can be viewed by clicking the lock icon near the edge of a browser window when accessing a secure site (secure sites use SSL encryption as seen by the "https://" prefix in the URL).
Secure certificate information can be viewed in Internet Explorer and Firefox by double-clicking the lock icon located near the bottom-right corner of the browser window. To view the certificate in Safari, click the lock icon located near the top-right corner of the browser window.