Windows Server can be one of three kinds: Active Directory "domain controllers", Active Directory "member servers" and Windows Workgroup "stand-alone servers".[8] The term "Active Directory Server" is sometimes used by Microsoft as synonymous to "Domain Controller"[9][10][11][12][13] but the term is discouraged.[14]
https://en.wikipedia.org/wiki/Domain_controller
Active Directory is what is called a directory service, it stores objects like users and computers. So you can consider it as as database that store users and computers configuration in AD domain.
A domain controller is the server running Active Directory; Domain controllers are typically referred as DC. Domain controller is a server based on MS windows Server 200X which is responsible for allowing host access to domain resources.
A Domain controller authenticates the users and the computers to join the domain. You can have many Domain controllers in your AD for many reasons, like redundancy and load balance as users can use anyone of them as they are replicating AD database.
Member servers are servers running within a domain. Member sever runs an operating system which belongs to a domain and is not a DC. Member server typically run different services on the machine can act like a file server web server application server print server.
https://social.technet.microsoft.com/Forums/windowsserver/en-US/60e064b5-5c6f-4c96-9abb-e3e51f4265c9/ad-server-vs-domain-controller-vs-member-server-et-al?forum=winservergen