Active Directory is a directory service that stores information about a computer network and allows centralized management. It provides features like hierarchical organization, a distributed database, scalability, security, and flexibility. When deploying Active Directory, it is important to plan the domain structure and verify the file system is using NTFS. Windows Server 2016 supports domain and forest functionality levels that determine available features. New features in Windows Server 2016 Active Directory include privileged access management, Azure AD join, and Microsoft Passport. Read-only domain controllers allow read-only access to Active Directory in less secure locations. Prerequisites like server hardware requirements must be met before installing Active Directory.