X.509 is a standard that defines the format of public key certificates. The standard is based on ASN.1.
An X.509 certificate consists of a public key and an identity such as a hostname, an organization or an individual. This data is signed by a certificate authority (CA) or is self-signed.
X.509 certificates are used, for example, in SSL/TLS
X.509 can be contrasted to the PGP's web of trust. The former uses a hierarchical approach based on a certificate authority.