–> The following four built-in database schemas cannot be dropped:
1. The “dbo” schema: is the default database schema for new objects created by users having the db_owner or db_ddl_admin roles. The dbo schema is owned by the dbo user account. By default, users created with the CREATE USER Transact-SQL command have dbo as their default schema.
2. The “guest” schema: is used to contain objects that would be available to the guest user. This schema is rarely used.
3. The “sys” schema: is reserved by SQL Server for system objects such as system tables and views.
4. The “INFORMATION_SCHEMA” schema: is used by the Information Schema views, which provide ANSI standard access to metadata. This schema is contained in each database. Each information schema view contains metadata for all data objects stored in that particular database. Information schema views are based on sys catalog view definitions.
… notes from: Training Kit (Exam 70-461) Querying Microsoft SQL Server 2012