The schema is the component of the Active Directory directory service that defines all the objects and attributes that the directory service uses to store data.
Active Directory stores and retrieves information from a wide variety of applications and services. So that it can store and replicate data from a potentially infinite variety of sources, Active Directory standardizes how data is stored in the directory. By standardizing how data is stored, the directory service can retrieve, update, and replicate data while ensuring that the integrity of the data is maintained.
The directory service uses objects as units of storage. All objects are defined in the schema. Each time that the directory handles data, the directory queries the schema for an appropriate object definition. Based on the object definition in the schema, the directory creates the object and stores the data.
The Active Directory Schema can be dynamically extensible, meaning that you can modify the schema by defining new object types and their attributes and by defining new attributes for existing objects.