"in human terms"
Not quite sure what you mean. Do you mean "less technical"? Not sure if that is easily done, but how about this?
In Windows NT, you gain access to resources on the network by creating shares -- such as shared folders and shared printers. You can then access those resources by mapping to them (using the \\servername\sharename UNC path convention), or by "browsing" the network. For example, if you click on Network Neighborhood (or "My Network Places") you see a "browse list" of all the computers on the network. Click on the icon for a particular computer, and you see any public shares (folders, printers) available on that computer.
In Active Directory, things are organized differently. In addition to the "Network" icon, you now have an "Active Directory" icon. Click on it, and you will see icons for all the domains in the directory. Click on their icons, and you will see the "objects" in those domains -- folders representing OU's (organizational units), domain controllers, computers, and other resources.
Or maybe you are asking about the Active Directory hierarchy? From the top down: Forests, Trees, Domains, OU's. If this is what you are asking about, and want more info, say so.