Plains and plateaus are different from mountains in that they are made up of rock formations that are in the same horizontal position - they are flat!
Plains are flat surfaces at low levels. Coastal plains are made up of bits of rock that are carried along from rivers to the ocean or are worn away from rocks along the seashore. Sometimes inland plains are formed when seas or lakes get filled in with sediment or soil and become flat plains.
Plateaus are raised areas of land with a flatter top. It differs from a mountain in that a mountain has a jagged peak at the top. Plateaus form either when mountains get worn down and their tops are not as jagged or when a large portion of flat earth is pushed up from the earth.
There are several special kinds of plateaus. One is a butte. A butte is like a plateau but its top is a bit more rounded. A mesa is a plateau with quite a flat top.