Schema 
You can use schemas to group database objects that logically belong together. A database object can belong to precisely one schema.
Schemas are created in the following situations:
When you create a new database, the database system creates the database system administrator and implicitly creates a schema with the same name. The database system also creates the following schemas for the database system administrator: DOMAIN, SYSINFO.
When you create a new database user, the database system creates a schema with the name of the database user.
Database administrators can create schemas explicitly.
More Information
SQL Tutorial, Schemas
SQL Reference Manual, Schema Name (schema_name)
Database Studio, Creating Schemas, Deleting Schemas
Database Administration in CCMS, Schemas