A CER - Internet Security Certificate is used by various websites to establish the validity of a particular website. Third parties such as Thwate and VeriSign commonly provide these certificates to online retail websites and other websites that wish to guarantee their users that the site is fully secure. Those who want to see CER files can double click on the lock icon near the bottom of an IE or Firefox browser window. Safari users should instead on the lock icon near the top right hand corner of the browser window.
The importance of this particular certificate goes without saying. It guarantees a user's privacy and the safety of his or her financial information and a user should always take the time to verify that the certificate presented by the website in question is bona fide.