Basic Active Directory Components

At its core, Active Directory needs structure to work properly. It provides the basic building blocks for people to build their own directory. These basic building blocks of Active Directory include domains, domain controllers, trusts, forests, organizational units, groups, sites, replication, and the global catalog.

Understanding Forests
At the top of the Active Directory structure is a forest. A forest holds all of the objects, organizational units, domains, and attributes in its hierarchy. Under a forest are one or more trees which hold domains, OUs, objects, and attributes.
As illustrated in this image, there are two trees in the forest. You might use a structure like this fororganizations with more than one operating company.
You could also design a structure with multiple forests, but these are for very specific reasons and not common.

Domains
At the heart of the Active Directory structure is the domain. The domain is typically of the Internet naming variety (e.g. Learnthat.com), but you are not forced to stick with this structure – you could technically name your domain whatever you wish.
Microsoft recommends using as few domains and possible in building your Active Directory structure and to rely on Organizational Units for structure. Domains can contain multiple nested OUs, allowing you to build a pretty robust and specific structure.

Domain Controllers
In Windows NT, domains used a Primary Domain Controller (PDC) and Backup Domain Controller (BDC) model. This had one server, the PDC, which was “in charge” while the other DCs where subservient. If the PDC failed, you had to promote a BDC to become the PDC and be the server in charge.
In Active Directory, you have multiple Domain Controllers which are equal peers. Each DC in the Active Directory domain contains a copy of the AD database and synchronizes changes with all other DCs by multi-master replication. Replication occurs frequently and on a pull basis instead of a push one. A server requests updates from a fellow domain controller. If information on one DC changes (e.g. a user changes their password), it sends signal to the other domain controllers to begin a pull replication of the data to ensure they are all up to date.
Servers not serving as DCs, but in the Active Directory domain, are called ‘member servers.’
Active Directory requires at least one Domain Controller, but you can install as many as you want (and it’s recommended you install at least two domain controllers in case one fails).

Trust Relationships
Trust Relationships are important in an Active Directory environment so forests and domains can communicate with one another and pass credentials. Within a single forest, trusts are created when a domain is created. By default, domains have an implicit two-way transitive trust created. This means each domain trusts each other for security access and credentials. A user in domain A can access resources permitted to him in domain B while a user in domain B can access resources permitted to her in domain A.
AD allows several different types of trusts to be created, but understanding the two-way transitive trust is the most important to understanding AD.

Organizational Units
An Organizational Unit (OU) is a container which gives a domain hierarchy and structure. It is used for ease of administration and to create an AD structure in the company’s geographic or organizational terms.