In cryptography, a public key infrastructure (PKI) is an arrangement that provides for trusted third party vetting of, and vouching for, user identities. It also allows binding of public keys to users. This is usually carried out by software at a central location together with other coordinated software at distributed locations. The public keys are typically in certificates.
The term is used to mean both the certificate authority and related arrangements as well as, more broadly and somewhat confusingly, the use of public key algorithms in electronic communications. The latter sense is erroneous since PKI methods are not required to use public key algorithms.
From: Wikipedia / Public-key infrastructure (http://en.wikipedia.org/wiki/Public_key_infrastructure)