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 a CA implementation but there is other implementation.
A PKI consists of:
The usual implementation is the certificate authorities one.
A Public key infrastructure (PKI) using the hierarchical system of Certificate authorities brings trust to the transactions by signing certificate.
On the contrary with the previous pki, the web of trust model is a decentralized model.