The electric field lines in a parallel plate capacitor are parallel wrt each other. But near the edges of the parallel plates, the charge distribution is non-uniform and hence the field lines are bent.. this effect is called fringe effect...
A similar thing happens between any 2 metal layers in the layout. The places where the plates are planar-parallel, u get uniform electric flux which will lead to coupling capacitances. the regions where the electric lines of force experience fringe, contribute to the fringe effect.
For eg, between a metal1 of 2u width and a metal2 of 1u width running in parallel can cause fringe cap...
hope this helps..