Photoshop :: Invisible Grid Over Image?

Just come back to a piece of work afte my son was playing with the computer, and he's set up something in Photoshop that I've never seen before. Really strugling to describe it – there seems to be an invisble grid or chequerboard over my image somehow. When I try to paste another image into a new transparent layer (I'm trying to combine two images), it doesn't appear solid but has transparent squares all over it. Also, if I try to select a block of colour with the magic wand tool, the marching ants go into a chequerboard shape. In the hope of un-setting this I've taken the history way back to beore I last worked on the image, but no joy – it's some kind of option that will have to be switched off. I'm sue it's something incredibly simple and obvious, it's just that I can't find it!