To quote the wikipedia:

X.509 is part of the hierarchial X.500 standard and thus assumes a strict hierarchial system of certificate authorities (CAs) for issuing the certificates. This is in contrast to web of trust models, like PGP, where everyone may sign the keys of others. The X.500 system have never been fully implemented, so the IETF's public-key infrastructure working group have made extensive updates to the standard in order to make it work with the more loose organization of the Internet. In fact today X.509 certificate usually refers to the X.509 v3 certificate specified in RFC2459.