I dont quite know how i'm going to describe it but here goes!!
They're square bits of plastic that vary in size and colour. You start with a large square at the bottom and put a smaller one on top each time, you then put a diamond like piece on the top, which makes it look like a pyramid. In the box you get a pack of cards showing you the different shapes you can make.
I'm guessing you'd get them in craft fayres, i've never seen them in toy shops.
If anybody knows what they are and where i can buy them from i'd be most grateful.