A valley is a long trough in the landscape that has been eroded by a river or a combination of river and glacier action.

A V-shaped valley.

Valleys that have been formed by river action tend to have straight slopes. In hilly country these slopes run right down to the riverside as shown in the picture above. This shape has given rise to the term "V-shaped" valley to describe them.

By contrast a valley that has been modified by a glacier (glaciers rarely create valleys) will also have steep sides, but it will have a wide flat floor, far wider than a river could make, hence the use of the term "U-shaped" valley to describe them.

The differences in valley shapes are much clearer in mountain regions than in lowland regions.

Yosemite valley in California is a U-shaped valley.