A cliff is a mass of rock that rises very high and is almost vertical, or straight up-and-down. Cliffs are very common landscape features. They can form near the ocean (sea cliffs), high in mountains, or as the walls of canyons and valleys. Waterfalls tumble over cliffs. Cliffs are usually formed because of processes called erosion and weathering. Weathering happens when natural events, like wind or rain, break up pieces of rock.

In coastal areas, strong winds and powerful waves break off soft or grainy rocks from hardier rocks. The harder rocks are left as cliffs. The tiny pieces of rocks broken off by weathering are called sediment or alluvium. Erosion is the process of transportation of this sediment.

On sea cliffs, sediment becomes part of the seafloor and is washed away with the waves. On inland cliffs, sediment is often carried away by rivers or winds. Larger rocks broken off by sediment are called scree or talus. Scree builds up at the bottom of many inland cliffs as rocks tumble down. These piles are called scree slopes or talus piles. Some scree slopes can be so large that soil and sediment can build up between the rocks, allowing trees and other vegetation to grow on the slope.

Most scientists and mountaineers think the Rupal Flank of Nanga Parbat, a mountain in the Himalayas, is the highest cliff in the world. The Rupal Flank rises 4,600 meters (15,092 feet) above its base. Others say the highest cliff in the world is the east face of Great Trango, in the Karakoram mountain range, which is 1,340 meters (4,396 feet) tall and one of the most difficult rock-climbs in the world. Both Nanga Parbat and Great Trango are located in Pakistan.