Active Directory is a Directory Service created by Microsoft. It is included with most Windows Server operating systems.
Almost all Active Directory installations actually include several separate but related components; although the term "Active Directory" technically refers only to the directory service, in general use it refers to the entire constellation of parts.
Active Directory is primarily used to store directory objects (like users and groups) and their attributes and relationships to one another. These objects are most commonly used to control access to various resources; for instance, an Active Directory might contain a group which grants its members permission to log into a certain server, or to print to a specific printer, or even to perform administrative tasks on the directory itself.
Active Directory also provides a useful configuration management service called Group Policy, which can be used to manage computers which connect to the domain in order to install packages, configure software, and much more.
Most Active Directory installations have a few distinct parts which all work together:
- a directory database which stores the actual directory information
- a Kerberos key distribution center which helps manage user passwords and other security resources
- a DNS server, which maps IP addresses to hostnames and back
- a DHCP server, which grants dynamic IP addresses to hosts as they join the network
- one or more Global Catalogs, which cache parts of the directory database and help speed up some common queries
Also, Active Directory is designed in such a way that it can be run on multiple computers at the same time, which coordinate between themselves to ensure that their data is always consistent; this process is called "replication".