X.509 is a CCITT standard for the encoding of public keys. It is known in English as a `public key certificate'. Other than the format for the public key, the protocol or mechanism for authentication is essentially the same as the DSA/RSA mechanism.
X.509 certificates contain representations of a public key, and are signed by an entity known as a Certificate Authority. Since digital signatures cannot be repudiated, one can infer that if the Certificate Authority is trusted, then by extension, any certificate signed by such an authority can also be trusted. Many commercial authorities exist.