The schema is a blueprint of all the objects in a domain. When you create a new forest, a default schema contains definitions for users, computers, and domains. Because you can�t have multiple definitions of an object, you can have only one schema per domain.
The file schema.ini contains the default schema�s definition, as well as the initial structure for the file ntds.dit (which stores directory data). The %systemroot%\ntds directory contains the file schema.ini. The file is in plain ASCII format so that you can type it.