X.500 is an international standard that defines a type of directory service which can provide information about people, such as e-mail addresses and phone numbers. Many organizations throughout the world have made information available in X.500.

X.500 has two major components:

DUA (Directory User Agent) - a client program, or interface, that enables the user to query the available information. DUAs differ, depending on the type of X.500 access.

DSAs (Directory System Agents) - which contain the information. X.500 consists of many DSAs, each of which holds part of the total information available. If necessary, several DSAs will be searched to answer a user's query, but this is invisible to the user.