Secure Sockets Layer (SSL) is a deprecated protocol for establishing secure communication between computers.
SSL was deprecated in favor of TLS due to serious security flaws present in all versions of SSL.
Public Key Infrastructure (PKI) is a system that helps entities secure transactions and communications over networks. Commonly, it involves a trusted third party called a Certificate Authority (CA) which verifies ownership of a server’s public key by validating an entity’s certificate.
PKI’s most basic use is to verify the authenticity of public keys used to establish secure channels of communication.