A public key infrastructure (PKI) is the management and database system for:
of digital certificates and public key
A central problem with the use of public key cryptography is confidence/proof that a particular public key is authentic, in that it is correct and belongs to the person or entity claimed, and has not been tampered with or replaced by a malicious third party. In short,
public key infrastructure validate the ownership of a public key (associate a public key with an identity)
PKI is itself often used as a synonym for the hierarchical CA implementation but there is other implementation.
The X.509 Hierarchical Certificate Authority PKI uses a hierarchy of certificate authority that signs the certificate.
On the contrary with the previous pki, the web of trust model is a decentralized model that is defined in the OpenPgp standard
e.g., Simple public key infrastructure (SPKI)