some AD FSMO stuff PDC, Schema master, RID master, infrastructure master, domain naming master

http://www.windowsnetworking.com/articles_tutorials/Managing-Active-Directory-FSMO-Roles.html

I just quote his good point here
Forest root domain - Schema Master and Domain Naming Master on the same machine, which should also host the Global Catalog.


Every domain - PDC Emulator and RID Master on the same machine, which should have beefy hardware to handle the load.

Every domain - Never place the Infrastructure Master on a machine that hosts the Global Catalog, unless your forest has only one domain or unless every domain controller in your forest hosts the Global Catalog.
 
now in my own word:
schema master and DNM are unique to the forest
PDC,RID and IM are on the domain level.
 
Global Catelague ... GC ... I always struggling with this one... this one seems more important than others. it is set in AD site services, so it is site based or location/subnet/physical based