Active Directory (AD) is a directory service developed by Microsoft. It is a centralized database that stores and manages information about network resources, including users, groups, computers, and other network objects. Active Directory provides a hierarchical structure for organizing and managing these resources in a networked environment.

Some key features and functionalities of Active Directory include:

1. User Authentication: Active Directory enables user authentication, allowing users to log in to computers and access network resources using their AD credentials.

2. User and Group Management: Active Directory provides tools for creating, managing, and organizing user accounts and groups. It allows administrators to set permissions and access controls for different resources based on user and group memberships.

3. Single Sign-On (SSO): Active Directory supports Single Sign-On, which allows users to authenticate once and access multiple resources without the need to re-enter their credentials.

4. Group Policy Management: Active Directory includes Group Policy, which allows administrators to define and enforce security policies, settings, and configurations across the network.

5. Domain Services: Active Directory uses the concept of domains, which are logical units that define administrative boundaries within a network. Domains can be interconnected to form a domain hierarchy.

6. Replication and Redundancy: Active Directory supports replication, allowing directory data to be synchronized across multiple domain controllers for redundancy and fault tolerance.

Active Directory is primarily used in Windows-based environments and is a core component of Windows Server operating systems. It provides a centralized and scalable solution for managing network resources, user accounts, and security policies. Active Directory is widely used in enterprise environments for user authentication, access control, and resource management.