Active Directory is available in many organizational systems where they allow users to log in to multiple applications or services using their single credentials all across multiple service servers. For example, you can take the example of Microsoft's Azure. You can create 10 different web applications (services) to facilitate users. You don't want to create a separate log in system for every website, and so on... You use the Active directory to allow users to use their organizational credentials to sign in to all of these services...
ASP.NET allows you (at the project creation time) to select whether you want Individual accounts or Organizational accounts, under Organizational accounts, you get to use an Active directory (from some provider) such as Microsoft Azure's directory. Please read
this document[
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