Schema

Generally, a schema is a model, plan, or pattern that defines a particular language or structure.

• In Active Directory, the schema provides definitions for every type of object that can be included in a forest.
• In relational databases, schemas are used to define entities, relations, attributes, and other logical and physical elements of the database.
• In XML, schemas are used to define the structure of documents.

References:

AD:

• http://msdn.microsoft.com/en-us/library/windows/desktop/ms675085(v=vs.85).aspx
• http://www.activedirectorytutorial.net/active-directory-schema.html

Database:

• http://weblogs.asp.net/scottgu/archive/2006/07/12/Tip_2F00_Trick_3A00_-Online-Database-Schema-Samples-Library.aspx

XML:

• http://www.w3schools.com/Schema/schema_intro.asp