The AD DS schema is a set of definitions for all object types in the directory and their related attributes. The schema determines the way that all user, computer, and other object data are stored in AD DS and configured to be standard across the entire AD DS structure. Secured by the use of discretionary access control lists (DACLs), the schema controls the possible attributes to each object within AD DS. In a nutshell, the schema is the basic definition of the directory itself and is central to the functionality of a domain environment. Care should be taken to delegate schema control to a highly selective group of administrators because schema modification affects the entire AD DS environment.
Objects within the AD DS structure such as users, printers, computers, and sites are defined in the schema as objects. Each object has a list of attributes that define it and that can be used to search for that object. For example, a user object for the employee named Weyland Wong will have a FirstName attribute of Weyland and a LastName attribute of Wong. In addition, there might be other attributes assigned, such as departmental name, email address, and an entire range of possibilities. Users looking up information in AD DS can make queries based on this information, for example, searching for all users in the Sales department.
Extending the Schema
One of the major advantages to the AD DS structure is the ability to directly modify and extend the schema to provide for custom attributes. A common attribute extension occurs with the installation of Microsoft Exchange Server, which extends the schema, more than doubling it in size. An upgrade from Windows Server 2003 or Windows Server 2008 AD to Windows Server 2008 R2 AD DS also extends the schema to include attributes specific to Windows Server 2008 R2. Many third-party products have their own schema extensions as well, each providing for different types of directory information to be displayed.
Performing Schema Modifications with the AD DS Service Interfaces
An interesting method of actually viewing the nuts and bolts of the AD DS schema is by using the AD DS Service Interfaces (ADSI) utility. This utility was developed to simplify access to the AD DS and can also view any compatible foreign LDAP directory. The ADSIEdit utility, enables an administrator to view, delete, and modify schema attributes. Great care should be taken before schema modifications are undertaken because problems in the schema can be difficult to fix.
Source of Information : Sams - Windows Server 2008 R2 Unleashed