Previously known as Azure AD DS.

To provide features that might be required for legacy on-prem applications, etc.

Entra ID is modern solution.

Features

AD Domain join
AD Group policy
Legacy protocols (LDAP,kerberos/NTLM, etc)

Key Components

• Managed domain (Managed by MSFT)
• Sync (one way sync from Entra ID)
• Virtual Network (Resources can only interact with managed domain through virtual network (same network or network which has access to this network where managed domain is deployed))

references