>>>How would everyone recommend I learn Active Directory basics quickly?
>>
>>What are you planning to do with it?
>
>Thanks for the response Al
>
>We've had a Windows 2003 Server and a Snap appliance which has worked fine. Recently added a Server 2008 R2 which we installed as a separate domain (didn't know what else to do) and now suspect that Server 2003 and Server 2008 can be made to work together better but don;t really know how.

Things will probably run smoother if the servers are on the same domain.

You can add the 2008 server as either a "member" server (easier) to the existing 2003 domain, or you can add it as another domain controller (more difficult, requires preparing the existing 2003 domain).

http://technet.microsoft.com/en-us/library/cc708131%28WS.10%29.aspx
http://technet.microsoft.com/en-us/library/cc733027%28WS.10%29.aspx

If you've put a lot of effort into the 2008 machine's current domain (created or purchased security certificates, etc.) that configuration effort or certificates may become useless or obsolete.