DBA and SYSDBA - Tips to a newbie(3)

SYSDBA and DBA are often misunderstood. In short, SYSDBA is a system privilege whereas DBA is a role.
The DBA role does not include the SYSDBA or SYSOPER system privileges. SYSDBA and SYSOPER are special administrative privileges that allow an administrator to perform basic database administration tasks, such as creating the database, instance startup and shutdown, drop a database, open and mount a database, place database in archivelog mode or remove it from archivelog mode.
About SYSDBA system privilege

The SYSDBA and SYSOPER system privileges allow access to a database instance even when the database is not open. Control of these privileges is totally outside of the database itself. The SYSDBA and SYSOPER privileges can also be thought of as types of connections that enable you to perform certain database operations for which privileges cannot be granted in any other fashion. For example, if you have the SYSDBA privilege, you can connect to the database by specifying CONNECT AS SYSDBA

courtesy :

http://momendba.blogspot.com/2007/07/closer-look-at-sysdba-and-dba.html