A certificate authority or certification authority (CA) is an entity that issues digital certificates for use by other parties. It is an example of a trusted third party. CAs are characteristic of many public key infrastructure (PKI) schemes.
There are many commercial CAs that charge for their services. There are also several providers issuing digital certificates to the public at no cost. Institutions and governments may have their own CAs.
More information is also available at Wikipedia - Certificate_authority