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 specific public key does in fact belong to a specific individual.
Certificates help prevent someone from using a phony key to impersonate someone else.
In some cases it may be necessary to create a chain of certificates, each one certifying the previous one until the parties involved are confident in the identity in question. 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.