Doug is correct for most playing cards out there. The black layer you see in the middle when you tear a card is not paper or cardboard, it is a dark glue that serves two purposes - Holding the paper together and making the cards opaque so that the value is not visible from the other side of the card.
Finish
Ink
Paper
Glue
Paper
Ink
Finish
This does vary, but it is the standard. Bikes and most custom cards are this way. If, by chance, you pick up a deck of Absolut Vodka playing cards, they are only a single layer of paper, which means no glue. Finish, ink, paper, ink, finish.
L