It's called Coroplast here in the states, and goes by a lot of different names overseas like Correx and Coroflute. It looks like cardboard, only it's plastic...and you see it everywhere and don't even know it. Here's some examples:
http://members.cox.net/deantuinstra/Signs/Signs.html

It can also be purchased from a sign shop or on-line distributer in 4' x 8' sheets for @ $10/sheet

There is a LOT more info here:
http://mywebpages.comcast.net/TipsTr.../tip_index.htm

Some guys fly these exclusively, some guys use these for beaters to get losened up for their beautiful Balsa planes, some guys beat them to a pulp learning funfly moves so they wont trash a beautiful FunFly bird, and some guys are really getting good at making Coroplanes look good! Some guys love em, some guys hate em...but the bottom line...is this is an awesome hobby and we're having one heck of a lot of fun!