SYS
When you create an Oracle Database, the user SYS is automatically created and
granted the DBA role.
All of the base tables and views for the database data dictionary are stored in the
schema SYS. These base tables and views are critical for the operation of Oracle Database. To maintain the integrity of the data dictionary, tables in the SYS schema are manipulated only by the database. They should never be modified by any user or database administrator, and no one should create any tables in the schema of user SYS. (However, you can change the storage parameters of the data dictionary settings if necessary.) Ensure that most database users are never able to connect to Oracle Database using the SYS account.

SYSTEM
When you create an Oracle Database, 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 Database options and tools. Never use the SYSTEM schema to store tables of interest to nonadministrative users.

The DBA Role
A predefined DBA role is automatically created with every Oracle Database installation. This role contains most database system privileges. Therefore, the DBA role should be granted only to actual database administrators.
Note: The DBA role does not include the SYSDBA or SYSOPER system privileges. These are special administrative privileges that allow an administrator to perform. basic database administration tasks, such as creating the database and instance startup and shutdown. These system privileges are discussed in "Administrative Privileges" on page 1-12.

理解要点：
--sys用户是建库的时候oracle自己建立的，主要是放了基表，动态和静态的视图，数据字典等。确保不要在sys schema下建表、不要让用户使用sys帐户连接数据库。
--system 主要是oracle收集信息使用的用户，供oracle数据库自动化管理使用。

--DBA角色不包括the SYSDBA or SYSOPER 系统权限.