here,database role i understood but my doubt is

1) what is schema

2) how to remove default schema

3) and also how to remove "CHECK's"  on Owned Schema Objects.

1. A schema is a kind of organizational unit to group object like tables, e.g. a schema "Finance" with all table for financial booking system. Benefit: You can grant permissions to a schema, so that users of "finance department" can access all tables in schema "Finance"; see Securing SQL Server

2. It's not possible, every user do have a default schema.

3. You cannot directly "uncheck" the owned schema, instead you have to change the owner of the schema to a different

Ans 1

A database schema is a way to logically group objects such as tables, views, stored procedures etc. Think of a schema as a container of objects.

You can assign a user login permissions to a single schema so that the user can only access the objects they are authorized to access.

Schemas can be created and altered in a database, and users can be granted access to a schema. A schema can be owned by any user, and schema ownership is transferable.

Ans -2

The schema that is being dropped must not contain any objects. If the schema contains objects, the DROP statement fails.

Requires CONTROL permission on the schema or ALTER ANY SCHEMA permission on the database.

http://technet.microsoft.com/en-us/library/ms186751.aspx

Ans 3

The database principal owns a schema in the database, and cannot be dropped