Certificate Authority (CA) is an organization that is trusted to sign digital certificates. CA verifies identity and legitimacy of company or individual that requested a certificate and if the verification is successful, CA issues signed certificate.
When server presents certificate to client (for example, your web browser) during SSL handshake, client will attempt to verify signature against a list of ‘known good’ signers. Web browsers normally come with lists of CAs that they will implicitly trust to identify hosts. If the authority is not in the list, as with some sites that sign their own certificates, the browser will alert the user that the certificate is not signed by a recognized authority and ask the user if they wish to continue communications with unverified site.
Comodo, GeoTrust and VeriSign Certificate Authorities are recognized by 99% of web browsers. This is the guarantee that browsers will not show any security errors when connecting to the site secured by Comodo, GeoTrust, or VeriSign.