Plains are flat surfaces at low levels, while plateaus are flat surfaces on top of a raised area. Plateaus often form out of mountains that are worn down at the top to be flatter.
Over the course of millennia, rain and other environmental factors wear down a mountain with a jagged peak so that it is flat on top even though it still rises off the ground. Plateaus are further subdivided into additional types such as buttes and mesas. Buttes have a completely flat surface on top. Mesas are more rounded forms of plateaus. In contrast, plains are flat but at low elevation. Examples of plains in real life include the Russian Steppes and the Eurasian Plains.
In terms of human settlement, people favor plateaus when they wanted increased protection from invasion and natural elements. Colonizing peoples favor plains because access to rivers and streams is more important. Farmland is often easier to access on plains as well. It is hard to build any type of road or transportation network on plateaus, so groups that want to build roads favor plains over plateaus as well. Plains are more accessible and plentiful, so plains are often colonized over plateau areas which are hard to travel to, as they are usually remote.