With regard to hiking, a cairn is a pile rocks placed near a trail in lieu of man-made trail markers (e.g., wood signs, wood posts, plastic/metal tree emblems, painting on rocks, etc.). Further, they are used on ambiguous trails where hikers have been known to get lost. Staying on a trail marked primarily with cairns can be challenging because they only signal that you are near a trail and do not necessarily signal the direction that the trail is heading. Thus, effectively following them along a trail requires finding the next one in the sequence.
A trail marked by cairns is maintained by a community of hikers. The community of hikers can include non-profit organizations, county/state/federal government workers, and hikers passing by. Since they serve to reduce trail ambiguities, it is essential that hikers help maintain the ones they pass. Restoring cairns is especially important after inclement weather has blown them over or covered them up (e.g., snow, tree branches). Maintaining a cairn can be as simple as adding a rock to the pile.
Typically, cairns have larger rocks on the bottom with smaller rocks on top. In addition, a pile of tree branches may be used to block an incorrect trail path. The following pictures show some from hikes in Poudre Park, CO and Estes Park, CO.