Group Policy Objects (GPOs) can then be linked to OUs to centralize the settings for various users or computers across an organization.
When people say "Active Directory" they typically are referring to "Active Directory Domain Services." It is important to note that there are other Active Directory roles/products such as Certificate Services, Federation Services, Lightweight Directory Services, Rights Management Services, etc.
In most cases, a Domain Controller will hold a copy of the Global Catalog.
A Global Catalog (GC) is a partial set of objects in domains in a forest.
I find myself explaining some of what I assume is common knowledge about it almost daily.
This question will, hopefully, serve as a canonical question and answer for most basic Active Directory questions.
You can see that the child domain's name was prepended forest root domain's name. You can have disjoint namespaces in the same forest, but that's a whole separate can of worms for a different time.
It is directly searchable, which means that cross-domain queries can usually be performed on a GC without needing a referral to a DC in the target domain.
If a DC is queried on port 3268 (3269 if using SSL), then the GC is being queried.
Don't just give a one-line answer; explain why your answer is right, ideally with citations. It provides authentication and authorization mechanisms as well as a framework within which other related services can be deployed (AD Certificate Services, AD Federated Services, etc).
Answers that don't include explanations may be removed. It is an LDAP compliant database that contains objects.