This week's book giveaway is in the Performance forum. We're giving away four copies of The Java Performance Companion and have Charlie Hunt, Monica Beckwith, Poonam Parhar, & Bengt Rutisson on-line! See this thread for details.
Dual is a place holder, nothing more. It exists as a table containing one column called dummy with the value x. You use it to return constant values, such as the value of a sequence. The reason you use dual is it is guarenteed to only ever have one row and it is visible to all users.