Watching Video, you have a solid understanding of PKI, but increasing formality : here's a formal answer consisting of LDAP, and RA besides CA and Certificates.

Merike Kaeo, quoting the Internet X.509 Public Key Infrastructure PKIX defines public key infrastructure (PKI) as the set of hardware, software, people, policies, and procedures needed to create, manage, store, distribute, and revoke certificates based on public key cryptography. PKI automate all these activities. PKI works best when there is a large mass of users. Under such circumstances, it creates and distributes digital certificates widely to many users in a trusted manner. It is made up of four major pieces: the certificates that represent the authentication token; the CA that holds the ultimate decision on subject authentication; the registration authority (RA) that accepts and processes certificate signing requests on behalf of end users; and the Lightweight Directory Access Protocol (LDAP) directories that hold publicly available certificate information. coction as the cryptographic proof

In large networks with varying communication topologies where network communicating elements cannot belong to the same KDC, key distribution becomes a real problem. These problems are solved when a Public Key Infrastructure (PKI) is used instead of KDCs to provide trusted and efficient key and certificate management.