SYS, SYSTEM and DBA

What is the difference between SYS, SYSTEM and DBA?

[B]>>What is the difference between SYS, SYSTEM and DBA?[/B]

[B][U]SYS[/U][/B]
-The Oracle user SYS owns all base tables and user-accessible views of the data dictionary.
-SYS is the superuser or like a 'root' in a oracle database.
-No Oracle user should ever alter (update, delete, or insert) any rows or schema objects contained in the SYS schema, because such activity can compromise data integrity.

[B][U]SYSTEM[/U][/B]
-When a database is created, the user SYSTEM is also automatically created and granted the DBA role.
-The SYSTEM username is used to create additional tables and views that display administrative information, and internal tables and views used by various Oracle options and tools.
-can't shutdown and startup the database.

[B][U]DBA[/U][/B]
It is a predefined role and is automatically created with every oracle database.

SYS : It owns the data dictionary. It has sysdba privilege which SYSTEM doesn't. It is a powerful user.Unless you connect as sysdba, it won't allow to shut the database. Donot create tables in SYS schema, because the objects under SYS user cannot be exported.

SYSTEM : It is a administration user, it possess tables other than Data dictionary.

SYS and SYSTEM are default users.