Public Key Infrastructure (Digital Cert )

A public-key infrastructure (PKI) is a set of hardware, software, people, policies, and procedures needed to create, manage, distribute, use, store, and revoke digital certificates

A digital certificate is a digital form of identification, much like a passport or driver's license. A digital certificate is a digital credential that provides information about the identity of an entity as well as other supporting information. A digital certificate is issued by an authority, referred to as a certification authority (CA). Because a digital certificate is issued by a certification authority, that authority guarantees the validity of the information in the certificate. Also, a digital certificate is valid for only a specific period of time.

Digital certificates provide support for public key cryptography because digital certificates contain the public key of the entity identified in the certificate. Because the certificate matches a public key to a particular individual, and that certificate's authenticity is guaranteed by the issuer, the digital certificate provides a solution to the problem of how to find a user's public key and know that it is valid. These problems are solved by a user obtaining another user's public key from the digital certificate. The user knows it is valid because a trusted certification authority has issued the certificate.

In addition, digital certificates rely on public key cryptography for their own authentication. When a digital certificate is issued, the issuing certification authority signs the certificate with its own private key. To validate the authenticity of a digital certificate, a user can obtain that certification authority's public key and use it against the certificate to determine if it was signed by the certification authority.

Reference:
http://technet.microsoft.com/en-us/library/bb123848(v=exchg.65).aspx