A schema is a container that holds tables, views, procedures, and so on. It is inside a database, which is inside a server. These entities fit together like nested boxes. The server is the outermost box, and the schema is the innermost box. It holds all of the securables listed below. But it cannot hold another box.
Securable that must be inside a schema
XML Schema Collection
XML SCHEMA COLLECTION
Every securable in a specific schema must have a unique name. The fully-specified name of a securable contained by a schema includes the name of the schema that contains it. Thus, a schema is also a namespace.
In SQL Server 2000 and earlier versions, databases could contain an entity called a "schema", but that entity was effectively a database user. In SQL Server 2005 and SQL Server 2008, a schema is both a container and a namespace.