System zoning, as described on System Zones, can also be applied to a single PCB.
Noisy parts of a system, like a digital circuit or a switch mode power supply, should be made as small as possible, reducing the size of current loops that will act as emitting antennas. Similarly, sensitive parts of a system, like an analog measurement circuit, should be made as small as possible, reducing the size of current loops that will act as receiving antennas. And of course, the noisy part of a system should be kept as far away from the sensitive ones as possible.
Remember that in both cases the important part is reducing the size of the current loops, not the physical board area. Routing in ground planes to save space should therefore be avoided, unless thorough analysis shows that the ground return paths of other signals will not be affected.