The AWS Directory Service uses Microsoft Active Directory and is referred to as AWS Managed Microsoft AD. As the name implies this is provided as a Managed Service. This is a huge advantage because everybody is already familiar with Active Directory and it is pretty much an industry standard.
It enables you to use to manage access to AWS Cloud resources using Active Directory with in-house AD credentials. Administration is performed using standard AD administration to manage Resources, Users Group Policy and single sign-on (SSO).
Effectively AWS becomes an extension of your domain. Amazon EC2 and Amazon RDS for SQL Server instances can be then be joined to this domain. AWS Enterprise IT applications such as Amazon WorkSpaces can be used with Active Directory users and groups.