If the logical components aren't difficult enough to keep straight, the physical components only add to the mix:
Sites—Groupings of computers (or subnets) that share high-speed, high-bandwidth connectivity. A subnet can be associated with only a single site. Sites contain only resource objects, such as computers. A site definition is stored as a site object in Active Directory. Multiple site objects construct a site topology, which is different from the logical topology of the network. This means that you can have one domain across more than one site, and you can have one site that contains multiple domains. Therefore, sites ignore domains and namespaces. They define how and perhaps when replication between DCs occurs, and which DC a user's computer contacts for authentication. A site can also have a Group Policy imposed upon it. Lastly, clients are associated with a site based on the subnet on which that client resides. Clients should first try to contact DCs within their site.
Domain controllers (DCs)—Servers that provide Active Directory services to clients and users. The DC stores a complete copy of the domain's objects, along with the Active Directory schema and configuration, in what are called partitions or naming contexts. Remember, the DC stores domain partition information only for the domain to which it belongs, but it stores schema and configuration partition information for the entire Active Directory. Each DC can write to Active Directory. Active Directory uses a multimaster replication model in which every DC is equal. Some DCs within the forest play an additional role in Active Directory. Some DCs play the role of Flexible Single Master of Operations (FSMO), which must be performed by a single DC in the domain or forest. The PDC Emulator is an example of an FSMO. When running in mixed mode, each domain can have a only single PDC Emulator. By default, the first DC in the domain assumes this role.
Global Catalog (GC)—Another partition that is replicated within Active Directory. However, this partition does not automatically exist on every DC the way that the domain, schema, and configuration partitions do. The GC partition is replicated only to DCs that have been designated as GC servers. GC servers are DCs that contain this added partition. They reply to requests from computers for Active Directory information about objects both from inside and outside of their domain.