• SYS and SYSTEM accounts have the DBA role granted to them by default
• SYS is granted SYSDBA privilege, SYSTEM is not
• To connect to the SYS account, you must use the AS SYSDBA clase
• SYS account in addition has all privileges with ADMIN OPTION
• SYS account owns the data dictionary
• SYS account owns the Automatic Workload Repository (AWR)
  
• SYS account owns the data dictionary
• Only users with SYSDBA or SYSOPER privilege, are allowed to start up and shutdown the database instance
  

For more on SYS and SYSOPER go to 6-12. System privileges

SYS and SYSTEM accounts are not used for routine operations. Create privileged users for that. For example, Jim has a low privilege account called jim and a privileged account called jim_dba. This method allows the principle of least privilege to be applied, eliminates the need for account sharing, and allows individual actions to be audited.