Character of DUAL:
DUAL is owned by SYS. SYS owns the data dictionary, therefore DUAL is part of the data dictionary. You are not to modify the data dictionary via SQL ever.
Dual table and its purpose:
Dual is just a convenience table. You don't need to use it, you can use anything you want. The advantage to dual is the optimizer understands dual is a special one row, one column table -- when you use it in queries, it uses this knowledge when developing the plan.
The Dual table structure, field and record details are below.
SQL> desc dual;
Name...
