Active Directory (AD) is an LDAP directory service released by Microsoft that uses the x.500 standard. Active Directory stores and manages various information and about an organization's resources from network information to computers, users and user permissions. The information in Active Directory is stored in units called objects. Active Directory was first release in Windows 2000.
A Directory Service is a directory of names, profiles, addresses and other resources in an environment. The service often allows different searches to be run against it to find users, machines or other resources. The information stored in the directory service is stored in a database. The information is often orgnized in hierarchical manner.
Lightweight Directory Access Protocol (LDAP) is a networking protocol used to access or modify a directory service information. LDAP, much like HTTP and FTP has its own standards and uses the prefix ldap:// as its URL. The latest version of LDAP as of July 2006 is LDAPv3.