A Barrow is a holy, consecrated burial place for Nord dead. In the case of larger barrows, they may contain not only the remains of many, many people, but also altars and offerings in accessible places and also statues, groupings of large burial urns and a variety of defensive traps. Virtually all barrows are defended by undead known as draugr. Some barrows are dedicated as resting places mainly of specific individuals, and perhaps warriors dedicated as draugr to defend that individual's resting place and their treasure.
Barrows are commonly assaulted by bandits and thieves, who, if they can handle the draugr infestation, will try to make it a stronghold for themselves.
The most elaborate barrows typically contain a Hall of the Dead, which features sculpted relief panels depicting heroic stories of the most important resident deceased. At the end of many such halls, there will be a circular puzzle door which will require the entrant to solve a combination using animal shapes. It is impossible to solve such a door without being in possession of a specific claw, which is made of a material unique to that particular door. See Dragon claw puzzles.
Said claw will not only fit into the door in question (into an obvious disc with appropriate depressions), but will have the solution to the puzzle imprinted on it, which requires examination (a click of the controller on the item in one's inventory) in order to view it. Solving the puzzle allows the entrant into the remainder of the barrows, granting access to the best of its treasure, and possibly a Word wall. All of this will typically be defended by a very powerful draugr, and sometimes even a Dragon Priest.