Dear Analog_IC,
Firstly, lets look into the fact that in a process, there are many variations in the wafer and in the diffusion. Because of these things, things like threshold voltage, mobility, diffusion resistance, bulk coefficient vary. Most importantly in the analog circuits you do not want the above because you are operating with real voltages and currents. We model these variations as the X-axis and Y-axis changes. In order to cancel these effects, we distribute the transistors(which are critical) in such a way that the mean variations cancel each other. Some of the common matching techniques are common centroid, interdigitization, etc.