How does this VM replace Microsoft Active Directory, on a network with Windows Machines on it?  Is there a specific component that is installed that replaces it?  Is it Samba or Webmin?  Are there any limitations in comparison to actual Active Directory? 
How does this Turnkey Linux domain-controller replace Microsoft Active Directory?
active directory;samba4;webmin;turnkey
The current Domain Controller container from Turnkey uses Samba, and includes the web-based interface (webmin). Samba is the component that replaces AD.It's difficult to say whether there are limitations compared to Microsoft AD; it depends on what components you are using from Microsoft. Generally speaking: no, there is no limitation, and in fact you might find SAMBA far more liberating, since it's a more flexible system than Microsoft's implementation. But obviously, it does depend on what you are used to, what you are expecting, and so on. Given that Turnkey is $0 to try, a trial run is probably the best determining factor.