An SSL Certificate is a popular type of Digital Certificate that binds the ownership details of a web server (and website) to cryptographic keys. These keys are used in the SSL/TLS protocol to activate a secure session between a browser and the web server hosting the SSL Certificate.
In order for a browser to trust an SSL Certificate, and establish an SSL/TLS session without security warnings, the SSL Certificate must contain the domain name of website using it, be issued by a trusted CA, and not have expired.
Browsers, operating systems, and mobile devices operate authorized CA ‘membership' programs where a CA must meet detailed criteria to be accepted as a member. Once accepted the CA can issue SSL Certificates that are transparently trusted by browsers, and subsequently, people and devices relying on the certificates.
Prior to issuing a Digital Certificate, the CA will conduct a number of checks into the identity of the applicant. The checks relate to the class and type of certificate being applied for.
For example, a domain validated SSL Certificate will have verified the ownership of the domain to be included within the Certificate, whereas an Extended Validation SSL will include additional information on the company, verified by the CA through many company checks.