The X.500 system is not part of the Internet protocols, but is available on the Internet. X.500 is the ITU Directory Service. This is a far more general information service than the DNS, and is targeted at providing White and Yellow page type information about organisations and people, as well as about computers. The BIG distinction between X.500 and DNS is that DNS is a simple lookup system, while the Directory is searchable.
Other important differences include the fact that the directory has an update protocol, and that it can be accessed securely (current work on DNS to add security/access control is not yet finished).
Otherwise, access to the directory and the structure of information in it (the Directory Information Tree) is typically quite similar to DNS. The general model is illustrated in 2.4.
Figure 2.4: Directory Services