A low lying relatively flat or slightly rolling land surface with very gentle slope and minimum local relief is called a plain. They can be classified as:

1. Structural plains: These plains are mainly formed by the uplift of a part of the sea floor or continental shelf. These are located on the borders of almost all the major continents. The south eastern plains of the United States formed by the uplift of a part of the Gulf of Mexico is an example of this type of plain.

2. Erosional plains: These plains are formed by the continuous and a long time erosion of all shorts of upland. The surface of such plains is hardly smooth. So these are also called peneplains which means almost plain. West Siberian plain is the example of this type 

3. Depositional plains: These are formed by the filling up of depression like lakes and seas by the sediments brought by rivers and other means. When plains are formed by rivers are called alluvial plains or riverine. The Indo-Gamgetic plain is a fine example of it.

The plains formed due to the deposition of sediments in the lake is called lacustrine plain or a lake plain. The valleys of Kashmir and Manipur plains are the examples.

When the plains are formed by the deposition of glaciers, they are called glacial plains.

When the plains are formed by the deposition of wind, they are called loess plains. Loess means air-borne fine dust. The Plain of North West China is an example of loss.