The Active Directory schema is a container that contains the definitions for all objects in the directory. Every new directory object you create is validated against the appropriate object definition in the schema before being written to the directory. The schema is made up of object classes and attributes. The base (or default) schema contains a rich set of object classes and attributes to meet the needs of most organizations, and is modeled after the International Standards Organization (ISO) X.500 standard for directory services. Because it is extensible, you can modify and add classes and attributes to the base schema. However, you should carefully consider each change you make, because extending the schema affects the entire network (forest and domain).
Systems Volume (Syvol)
In Microsoft Windows, the System Volume (Sysvol) is a shared directory that stores the server copy of the domain’s public files that must be shared for common access and replication throughout a domain. The term SYSVOL refers to a set of files and folders that reside on the local hard disk of each domain controller in a domain and that are replicated by the File Replication service (FRS). Network clients access the contents of the SYSVOL tree by using the NETLOGON and SYSVOL shared folders. Sysvol uses junction points—a physical location on a hard disk that points to data that is located elsewhere on your disk or other storage device—to manage a single instance store.