active directory Concepts

objects：An object refers to something concrete such as a user, a printer, or anetwork share. Objects have mandatory and optional attributes that describe them.

schema：Every object is an instance of a class defined in the schema. The schema defines the types and is itselfstored in objects in Active Directory.

Configuration：In addition to objects and class definitions stored as objects, the configuration of Active Directory itself isstored in Active Directory.

The active Directory Domain：A domain is a security boundary of a Windows network. In the Active Directory domain, the objects arestored in a hierarchical order. Active Directory itself is made up of one or more domains. 

Domain Controller：A single domain can have multiple domain controllers, each of which stores all of the objects in the domain.

Site：A site is a location in the network that holds at least one DC.

Domain Tree：Multiple domains can be connected by trust relationships. These domains share a common schema, acommon configuration, and a global catalog (more on global catalogs shortly). A common schema and acommon configuration imply that this data is replicated across domains. Domain trees share the same classand attribute schema. The objects themselves are not replicated across domains.

Forest：Multiple domain trees that are connected by using a common schema, a common configuration, and aglobal catalog without a contiguous namespace are called a forest. A forest is a set of domain trees.

Global Catalog:The GC is a read-only cache of all the objects that can be used only for searches; the domain controllersmust be used to do updates.