There are many components that make up Active Directory, and it is critical that you understand these components and the concepts surrounding them.
Active directory includes the rules, which helps in defining the classes of the objects contained in the directory. It can also define the constraints on the instances of the objects and guiding the arrangement of their names. Active directory also contains a global catalog which contains information about each and every objects present in the directory. The global catalog can help users to look for directory information regardless of the realm of the file. It also contains a query mechanism; therefore the properties of the objects can be located by the network users for various applications. Replication mechanism is a vital part of active directory which helps in distributing data throughout the network. All domain administrators participate in this process in order to receive the comprehensive copy of the directory information for their realm. Any change to the active directory information is automatically becomes the part of the replicated file.
Overview the attached below pdf file for visualizing the concept.