 <p>Well I think that an answer for your question exists somewhere in <a href="http://technet.microsoft.com/en-us/library/cc754361%28v=ws.10%29.aspx" rel="nofollow">Microsoft documentation</a>.</p> <p>Active-Directory is a bit more more than a common LDAP Directory like OpenLDAP or Oracle Directory Server Enterprise Edition (formerly SUN Directory Server Enterprise Edition). It's a "<strong>System</strong> Directory". The main difference with a simple Directory is that it's built in with a Microsoft specific "Schema" and a security infrastructure with "Kerberos". So installing AD is the same as promoting a server in a "Domain controler". In other words you can use the AD for your own usage, but his first usage is for the "System" (Domain) security.</p> <p>ADLDS is the most recent name for an old product called ADAM (Active Directory Application Mode). As you can guess, this Directory can be created just for the usage of your own application, you can build the Schema you want, you can install it on a Worksation or a server, it exists version that you can redistribute with your own application.</p>