AWS Directory Service is a series of directory services included Microsoft Active Directory in the cloud. There are numerous flavors: AD Connector, AWS Directory Service for Microsoft Active Directory (Enterprise Edition), Cognito & Simple AD
AD Connector and Microsoft Active Directory (Enterprise Edition) support standard Active Directory administration tools and take advantage of built-in Active Directory features such as Group Policy, trusts, and single sign-on.
AD Connector
AD Connector connects existing AD implementation with the AWS cloud which enables existing security polices like password expiration/history and account lockout with existing RADIUS-based MFA to manage resources like EC2 instances and applications like WorkSpaces, WorkDocs or WorkMail. AD Connectors and on-premises domains have a 1-to-1 relationship. There does not seem to be a limit to the number of users AD Connector supports.
Simple AD
Simple AD is a Microsoft Active Directory–compatible directory from AWS Directory Service using Samba 4 for use cases of less than 5000 users. This service does NOT support MFA or additional AD servers, support trust relationships with AD or transfer FSMO roles for AD management.
Microsoft Active Directory (Enterprise Edition)
Microsoft Active Directory (Enterprise Edition) is a feature-rich managed Microsoft Active Directory hosted on the AWS cloud. Microsoft AD is the best choice for more than 5,000 users and need a trust relationship set up between an AWS hosted directory and your on-premises directories.
Cloud Directory
Amazon Cloud Directory is a great choice when you need to build application directories such as device registries, catalogs, social networks, organization structures, and network topologies.
Cognito
You can also use Amazon Cognito when you need to create custom registration fields and store that metadata in your user directory. This fully managed service scales to support hundreds of millions of users fully supports federated identities.
Triage
Scalable? Cloud Directory
Have AD and wanna use it on AWS? AD Connector
More than 10 directories and 5 snapshots per each directory? Call AWS for limit increase.