Question

Im very new to Oracle so can anyone please explain about sys and system users?
And also I read these users are granted DBA role . Whether DBA and SYSDBA roles are same ?

Answer 1

(1)
http://www.adp-gmbh.ch/ora/misc/sys_system_internal.html

SYS

SYS is the owner of the database and the owner of the data dictionary.
Never ever create objects in the SYS schema.
The objects belonging to SYS cannot be exported.

SYSTEM

SYSTEM is a privileged administration user, and typically owns Oracle provided tables other than the dictionary. Don’t create your own objects under SYSTEM.

(2)
Not, these’re not same. There’s a big difference between them
For sysdba role, you can look to documentation

http://download.oracle.com/docs/cd/B19306_01/server.102/b14231/dba.htm#sthref137

And from documentation we see that:

The DBA role does not include the SYSDBA or SYSOPER system privileges

Additional differences between a user with the SYSDBA system privilege and a user with DBA role are that a SYSDBA authenticates via the password file and can edit tables in the SYS schema such as USER$, plus the SYSDBA connection will show as SYS and will be audited as SYS in mandatory audit. Of course most SYSDBA users have also been granted the DBA role as well. The difference with actual SYS is that they HAVE to logon as SYSDBA whereas a different DBA user granted SYSDBA system privilege can choose not to…

http://www.oracleforensics.com/wordpress/index.php/2008/09/21/bypassing-ora-01997/