A certificate authority, or CA, is a company that creates digital certificates. A digital certificate indicates that the named subject of the certificate owns the public key specified in the document. Others (relying parties) may depend on assertions or signatures regarding the private key corresponding to the certified public key.
PKI is a system for creating and distributing certificates—digital credentials that enable computers to exchange information with one another.
The PKI maintains a central repository of digital certificates, which map public keys to entities and are securely saved. If they are no longer valid, they may be revoked. The PKI provides trust so that anyone can rely upon the information provided by any certificate.
The goal of any PKI is to facilitate secure communication simply by using public-key cryptography.
One example would be securing communications between a web browser and web server using SSL/TLS, where public keys are exchanged during the initial handshake phase and are used for encryption until the end of the session.