I'm trying to explain it via the signs in the picture this time around, assuming the text is legible. A short description, though:
Redstone causes a block update, in a range of 1 block around it (not diagonally, but horizontally and vertically)
bug 108 is closely related to this, as once this one would be fixed, pistons would't work without update AT ALL (I think).
Edit: finally noticed 108 is resolved; WAI. Not sure if this one is an issue anymore, but I read it had good uses, and I'm kind of inspired. I have to admit this is complicated, though.