They are built for several purposes:
They vary from loose, small piles of stones to elaborate feats of engineering. In some places, games are regularly held to find out who can build the most beautiful cairn. Cairns along hiking trails are often maintained by groups of hikers adding a stone when they pass.
Starting in the Bronze Age, cists were sometimes interred into cairns, which would be situated in conspicuous positions, often on the skyline above the village of the deceased. The stones may have been thought to deter grave robbers and scavengers. A more sinister explanation is that they were to stop the dead from rising. It is noteworthy that there is a Jewish tradition of placing small stones on a person's grave whenever you visit, as a token of respect. (Flowers are not usually placed on graves in the Orthodox Jewish tradition.) Stupas in India and Tibet etc. probably started out in a similar fashion, although they now generally contain the ashes of a Buddhist saint or lama.
In Scotland, it is traditional to carry a stone up from the bottom of the hill to place on a cairn. In such a fashion, cairns would grow ever larger. An old Scots Gaelic blessing is Cuiridh mi clach air do chàrn, i.e. 'I'll put a stone on your cairn'. In the Faroe Islands (which are plagued by frequent fogs and heavy rain, and have some of the highest seacliffs in the world) cairns are a common navigational marker over rugged and hilly terrain. In North Africa, they are sometimes called kerkour. Cairns are also common on the Mediterranean island of Corsica.
Today, cairns are often used to mark hiking trails or cross-country routes in mountain regions at or above the tree line. Most are small, a foot or less in height, but a few are built taller so as to protrude through a layer of snow. It is traditional for each person passing by a cairn to add a stone, as a small bit of maintenance to counteract the destructive effects of severe winter weather. Often the habit is to only add to the top, and to use a smaller stone than the previous top stone, resulting in a precarious stack of tiny pebbles.
In Scandinavia, cairns are still used as sea marks. They are indicated in navigation charts and maintained as part of the marking system. To increase visibility they are usually painted white.
The Duan Eireanach, an ancient Irish poem, describes the erection of a family cairn; and the Senchus Mor, a collection of early Irish laws, prescribes a fine of three three-year-old heifers for "not erecting the tomb of thy chief."
Meetings of the tribes were held at them, and the inauguration of a new chief took place on the cairn of one of his predecessors. It is mentioned in the Annals of the Four Masters that, in 1225, the O'Connor was inaugurated on the cairn of Fraech, the son of Fiodhach of the red hair. In medieval times cairns are often referred to as boundary marks, though probably not originally raised for that purpose.
In a charter by King Alexander II of Scots (1221), granting the lands of Burgyn to the monks of Kinloss, the boundary is described as passing "from the great oak in Malevin as far as the Rune Pictorum," which is explained as "the Carne of the Pecht's fieldis."
In Scottish Highland districts small cairns used to be erected—even in recent times—at places where the coffin of a distinguished person was "rested" on its way to the churchyard. Memorial cairns are still occasionally erected, as, for instance, the cairn raised in memory of the prince consort at Balmoral, and "Maule's Cairn", in Glenesk, erected by the earl of Dalhousie in 1866, in memory of himself and certain friends specified by name in the inscription placed upon it.
The practice is common in English, cairns are sometimes referred to by their anthropomorphic qualities. In German and Dutch, a cairn is known as Steinmann and Stenenman respectively, meaning literally "stone man". A form of the Inuit inukshuk is also meant to represent a human figure, and is called an inunguak ("imitation of a person"). In Italy, especially the Italian Alps, a cairn is an "Ometto," or a "small man".
The Finnish name for a cairn used as sea mark is "kummeli" which is derived from the Swedish word "kummel".
In areas of ancient Dalmatia, as Herzegovina and Krajina, they are known by the Serbian word gromila.
In the mythology of ancient Greece, cairns were associated with Hermes, the god of overland travel. According to one legend, Hermes was put on trial by Hera for slaying her favorite servant, the monster Argus. All of the other gods acted as a jury, and as a way of declaring their verdict they were given pebbles, and told to throw them at whichever person they deemed to be in the right, Hermes or Hera. Hermes argued so skillfully that he ended up buried under a heap of pebbles, and this was the first cairn.
In English, however, structures in/below water are not generally called "cairns".