What is the dual table in Oracle?

It is a dummy table with one element in it. It is useful because Oracle doesn't allow statements like

 SELECT 3+4

You can work around this restriction by writing

 SELECT 3+4 FROM DUAL

instead.

It's a sort of dummy table with a single record used for selecting when you're not actually interested in the data, but instead want the results of some system function in a select statement:

e.g. select sysdate from dual;

See http://www.adp-gmbh.ch/ora/misc/dual.html