Certificates are digital documents attesting to the binding of a public key to an individual or other entity. They allow verification of the claim that a given public key does in fact belong to a given individual.
Certificates help prevent someone from using a phony key to impersonate someone else.
In their simplest form, certificates contain a public key and a name. As commonly used, a certificate also contains an expiration date, the name of the certifying authority that issued the certificate, a serial number, and perhaps other information. Most importantly, it contains the digital signature of the certificate issuer. The initial version of X.509 was published in 1988, version 2 was published in 1993, and version 3 was proposed in 1994 and considered for approval in 1995. Version 3 addresses some of the security concerns and limited flexibility that were issues in versions 1 and 2.