DUAL Table

System table of owner SYS, used to select one value from system function, user-defined function which return scalar value
SELECT SYSDATE FROM DUAL;

SYSDATE
---------
02-MAY-09

SELECT SYSDATE FROM ACCOUNT;
SYSDATE
---------
02-MAY-09
02-MAY-09
.......

SELECT fnGetAccountNameByAccountID(100000) FROM DUAL;
fnGetAccountNameByAccountID is a user-defined function to return scalar value (Account Name) based on AccountID parameter.