The valid attribute types and object classes that can appear in a directory. The attribute types and object classes define the syntax of the attribute values, which attributes must be present, and which attributes may be present for the directory.
The rules that define how the Directory tree is constructed. The schema define specific types of information that dictate the way information is stored in the Directory database.
In LDAP, the set of defined attributes and object classes that defines the content of acceptable entries within the directory server.