A valley is a hollow or surface depression of the earth bounded by hills or mountains, a natural trough in the earth's surface, that slopes down to a stream, lake or the ocean, formed by water and/or ice erosion. The form of a valley depends upon the rate at which deepening and widening goes on. V-shaped valleys are caused by forces such as erosion and rivers. Valleys that are not V-shaped were formerly occupied by glaciers and are characteristically U-shaped, formed by the huge bodies of ice that moved along.
Valley types. Source: Freie Universität Berlin
Plains and plateaus are broad, nearly level stretches of land that have no great changes in elevation. Plains are generally lower than the land around them; they may be found along a coast or inland. A plateau is a large highland area of fairly level land separated from surrounding land by steep slopes.
The Earth's slopes can be classified as three types according to their shape: straight slopes, concave slopes, and convex slopes. A slope consists of the three parts crest, free face, and talus slope.
Profile of a slope. Source: SA Geography.
The top line of a hill or ridge is also called crest (see picture above: A).
Landforms to which these definitions do not apply.