They are actually a textured surface that leaves an impression on the cake. My snowman cake was made with the diamond mat around the sides. They have alot of different styles-lines, dots, even cobblestone. You can use them against fondant or buttercream.
Hope that helped.
Angie
Here's a link with a few of the different kinds. They are usually on clear mats that are flexible.
http://www.cakesbysam.com/store/cart.php?target=search&substring=impression+mat
They make an impression on the cake...like ya know that 'quilted' look? Thats done with an impression mat...ya push it into the cake or for fondant, ya roll a rolling pin over it and mash it into the fondant...they're really cool and some are real spendy!!! 
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