The infrastruture required to verify that some user is who they say they are, either by PGP's WebOfTrust or a centralized certification infrastructure provided by a CertificationAuthority (or several). Also known as a PKI.

There are many parts to the infrastruture:

The security of the PublicKeyInfrastructure is not uniform and for many of the players issues such as ease of use, price and reliability are much more important.