An Active Directory (AD) (also known as Active Directory Domain Service, AD DS) is a "directory service" for Windows networks. This allows you to manage, for example, who has access to certain resources within the domain of a Windows network, such as shared folders or Remote Desktop servers.
The server that offers an Active Directory within a Windows network is called the 'Domain Controller' (DC). The DC authenticates and authorizes all users and computers within a Windows network.
An Active Directory uses (usually) a local domain (e.g. transip.local). For this reason, one also speaks of a 'Windows Domain' instead of a Windows network or Active Directory environment. This is also the reason that the server that offers the Active Directory is called a Domain Controller. Thanks to the use of a domain, the different servers know how to find each other.
Typically, a local domain that is not publicly accessible is used, so that an outsider can't query the domain's DNS records to determine which servers exist within the Windows domain.