Taken from Jerry Coyne's site:
The horizontal lines, astonishingly, are in fact straight. It's interesting to bring a ruler or other straight surface up to one of the lines: just before you get there - but only just before - you'll see the line straighten out. In fact you can see that on the very bottom line here, which does look straight. If it was raised just slightly it would look as curvy as all the rest.
The original cafe-wall illusion was about parallel straight lines between staggered rows of alternating black and white "bricks" appearing to be sloped. You can play around with it here. So strictly this is a variation on that illusion.
There's an attempt at an explanation here.