What Are Internal User Account? - Oracle DBA FAQ - Managing Oracle User Accounts, Schema and Privileges

An internal user account is a system predefined user account. Oracle 10g XE comes with a number of internal accounts:

• SYSTEM - This is the user account that you log in with to perform all administrative functions other than starting up and shutting down the database. SYSTEM is automatically created when you install the server. It's password is the one you specified during the installation process.
• SYS - This is another user account automatically created when you install the server. It's password is the one you specified during the installation process. All base tables and views for the database data dictionary are stored in the SYS schema. So avoid log in as user SYS as much as possible to reduce the risk of damaging those important data objects. User SYSTEM is preferred for all administrative tasks except starting up and shutting down.
• Other internal user accounts - Other special user accounts are predefined for special purposes. For example, CTXSYS is a special user account used by the Oracle Text product.