A rock shelter or abri is an overhanging cliff face, which is actually not what we would call a cave. The term rock shelter is pretty descriptive, the term abri is the French word for rock shelter which is often used in scientific papers. This is simply because in early 20th century, when the first important archaeological discoveries were made in rock shelters, they were concentrated to central France. So the term abri became an international scientific term.
A rock shelter is not at all what most people associate with a cave: a dark and confined space, where sunlight does not reach. But rock shelter actually do fit the definition of a cave (see Classification of Caves). However, rock shelters are not of speleological interest, but they are important archaeological excavation sites. That's simply because they are ideal shelters for nomadic people, offering a sort of weather protection, while still allowing light and warmth to enter, especially the ones facing south. And almost as important as that, the ceiling not only protects people but also their remains, it even covers them after some time with debris from the eroding cliff face and thus conserves them for millennia. This makes rock shelters the most important archaeological sites for prehistoric remains.
Rock shelters are formed by erosion. If there are weaker and stronger rocks, the weaker rocks will erode faster and form indentations. If the layers are horizontal the indentation will be a ledge. This is a common situation with all kinds of sedimentary rocks, so rock shelters are common in any kind of sedimentary rock. They are most common with sandstones and with interbedded limestones and marls.
A second kind of erosion forming rock shelters is the erosional force of the sea and of rivers. In both cases the force of the water works on the rock, and causes the normally horiozontal indentation just by concentrating its energy at the same point for quite some time. The kind of rock and its weakness are not important for this process. When the land is elevated from the sea, or when the river continues to cut deeper, the rock shelter will become a fine living place.
And finally there are lots of rock shelters formed by karst processes. They are the remains of eroded karst caves, cave ruins, which are often described as the last stage of cave development. A former cave, which was much bigger, is slowly destroyed by ongoing erosion. The ceiling collapses and only the concave walls remain of a vaulted dome. Often the former caves can be recognized as caves by the remains of speleothems.
The most important rock shelter of the world are located in France, the abris along the Vezere valley in the Perigord. This is the place where abris were discovered for archaeological research. Many locations are actually typesites or type locale for archaeological eras. Famous are also the cliff dwellings of the Mesa Verde National Park in the U.S.A. Then there are the important dwellings and cave entrances of the Lone and Blau valleys in southern Germany, where the oldest artworks of mankind were discovered.