This picture comes from the Aven d'Orgnac, a cave in the Ardèche region of south west France. The formation you see is know as l'escargot, as it resembles a snail (being ridden by a dwarf). It is a stalagmite (albeit a rather wonderfully shaped stalagmite), formed by the action of water as it drips down from the roof above. Rain water is naturally a very mild acid, which slowly dissolves limestone to form caves. Limestone is made of calcite, and the water transports the dissolved calcite with it as it goes. When the water evaporates the calcite is precipitated, and slowly over many thousands of years cave formations like the one below get built.
As a bonus, this is also a good example of pareidolia, the act of seeing patterns where there are none (such as faces in the clouds). It evolved as a defence mechanism: better to run away from a fake tiger than not notice a real one.