A cliff is a vertical rock face, or at least one that is very steep. Bare rock can always be seen on cliffs.

Cliffs are formed by waves beating against the bottom of the cliff, undercutting it, so that the top part of the cliff finally falls away under its own weight.

The shape of the cliff depends on the rock it is made from. The White Cliffs of Dover, for example, are made of chalk which has no lines of weakness, and is one thick band of the same rock. The cliffs at Whitby look more like a flight of stairs because they are formed by many thin bands of rock. Some bands of rock are hard and others soft. The hard bands stand out as thin ledges (the risers of the staircase) while the soft rocks wear back more easily (the treads of the staircase).

Cliffs can also be formed by glaciers as they scour away valleys. The sides are always very steep and part of what geographers call "U-shaped valleys'.