A Certificate Authority, or CA, is a highly trusted entity given the responsibility of signing and generating digital certificates. CAs are one of the most important pillars of a PKI.
After a CA signs and issues a certificate, that certificate can be used for establishing communication, or other tasks. If a certificate is issued for SSL, that certificate cannot be used for Secure Email. CA also verifies the owner of the certificate and checks if the certificate and revocation status are valid.