In addition to these users, a user can connect with diffrent levels of privileges, namely SYSDBA and SYSOPER. When you connect using "connect sys/passwd as sysdba" your connecting as the SYS user and requesting SYSDBA privs. Because the SYS user is the Oracle equivilent to the UNIX root user Oracle makes you specify the amount of control you have, which is why you'll get an error if you try to connect without specifying the privs:
SQL> connect sys/passwd
ERROR:
ORA-28009: connection to sys should be as sysdba or sysoper
SQL> connect sys/passwd as sysdba
Connected.
SQL>
The big diffrence between SYSDBA and SYSOPER privs is that SYSDBA can do anything (just like root). The SYSOPER privs allow you just about the same amount of control but won't allow you to look at user data. Both privs allow you to ALTER DATABASE, CREATE SPFILE, STARTUP or SHUTDOWN, ALTER DATABASE ARCHIVELOG, and includes RESTRICTED SESSION privs. However, only SYSDBA can CREATE or DROP DATABASE, and the ALTER DATABASE RECOVER options for SYSOPER are limited to complete recovery only.