Wikipedia
DigiCert Inc is a privately held, US based X.509 SSL certificate provider. As a trusted third party, DigiCert verifies the authenticity of secure websites on behalf of a web browser.
As a founding member of the CA/Browser Forum, the group that sets industry standards for publicly trusted digital certificates, DigiCert assisted in the development of the Extended Validation Certificate, which provides enhanced identity verification of certificate holders. DigiCert also worked in conjunction with Microsoft to develop and promote the use of subject alternate names in SSL certificates, for use with Microsoft Exchange Server.
DigiCert is the largest public CA that does not issue "domain validated", "low assurance", or "instant" SSL certificates either directly or through a subsidiary brand. It primarily sells Organization Validation (OV) and Enhanced Validation (EV) SSL certificates.
DigiCert Inc. is of no relation to Digicert Sdn. Bhd, a Malaysian-based certification authority that issues certificates with weak keys and had its trust revoked by web browsers.