A cairn is, in essence, an intentionally-laid of rocks, carefully stacked without mortar. Cairns are thought to have some meanings associated with marking property ownership or control, such as a landmark, a territorial marker, or a grave marker.
The word is occasionally used to mean a specific type of rock-covered burial; and as such are often only part of an archaeological site. Eventually, many rock-covered burials collected wind blown earth and are now classed as mounds.
Cairns are found throughout history and all over the world.
Sprague, Roderick. 2005. Burial Terminology: A guide for researchers. Altamira Press, Lanham, Maryland.
Rakita, Gordon F. M., Jane Buikstra, Lane A. Beck, and Sloan R. Williams(eds). 2005. Interacting with the Dead: Perspectives on Mortuary Archaeology fo the New Millennium. University Press of Florida, Gainsville.