What is the difference between SYS and SYSTEM database user accounts?

SYS

• automatically created when Oracle database is installed
• automatically granted the DBA role
• has a default password: CHANGE_ON_INSTALL (make sure you change it)
• owns the base tables and views for the database data dictionary
• the default schema when you connect as SYSDBA

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. Database users should not connect to the Oracle database using the SYS account.

SYSTEM

• automatically created when Oracle database is installed
• automatically granted the DBA role
• has a default password: MANAGER (make sure you change it)
• used to create additional tables and views that display administrative information
• used to create internal tables and views used by various Oracle database options and tools

Never use the SYSTEM schema to store tables of interest to non-administrative users.