What are poker table tops usually placed on?

I have been looking at poker tables and see that there are a lot of poker table tops. It seems like there are the rectangular and octagonal shaped tops. What are these supposed to rest on? is there a standard?
Examples: http://www.discountpokershop.com/poker-craps-and-roulette-folding-table-top-cup-holders-p-306.html

http://www.discountpokershop.com/2in1-pokerblackjack-table-top-p-19.html