Custom (tradition, ritual, обычай) is certain set of actions and believes that are commonly accepted in some society, but, perhaps, are not written explicitly. Also, this term includes the specific set of taboos, prohibitions, that are assumed to be obvious, but sometimes are not explained.
Customs and traditions
The customs include prohibition of certain actions. Often, such prohibitions are not written in Laws, but strongly "assumed".
Here is example of the folklore story about one poor boy and very rich daughter of one business man. The student wanted to marry her, but her father told, that it is "absolutely impossible". After few months, the daughter of the rich businessman become pregnant, and she indicated that student as father. The businessman changed his mind, invited the student and told him, that he should marry her. The student reminded that this is "absolutely impossible"..
In this example, the businessman and the student assumed different sets of taboos; and during the first conservation, they did not specified, did not declared them. The negation of some taboos may be qualified as hihilism  by those that consider those taboo as some absolute imperative; in this case, the statement "You are nihilist" means "Our opinions are different".
Customs as knowledge
Many customs are transfered as folklore, that includes legends (stories), proverbs, jokes, stories, music, dances and, perhaps, other rituals. Historically, the tribes with certain customs were more competitive than neighbors; in this sense, the customs appear as kind of knowledge, as information that helps in the practical activity.
In order to indicate place of customs, they should be separated from other kinds of knowledge – art, religion and science. As soon as the custom is well written, stored in a reproducible way, in becomes art; and if it fits some strict rules, that are assumed to be compulsory, it becomes religion.
For example, the story from the previous section becomes art, as soon as it is written and available for reproduction and critics; in particular, its artistic quality can be negated and disputed.
Different civilizations may have different customs. The attempts to force one tribe to follow customs of another tribe usually lead to the degradation. In particular, this refers to th custom to solve political questions in a democratic way, see Democracy for export.
Interpretation of custom as knowledge slightly narrows the set of meanings of this term. For example, the practice of consumption of enormous amount of alcoholic beverages of doubtful quality falls out of category of knowledge. (Although some alcoholics may be qualified as "professional drinkers".)
Customs as ethnicity
Customs are form ethnic individuality of the nation, tribe. Often, representatives of some civilization consider their customs as correct, "true", and other customs as "bad", "wrong". In this case, the difference in customs may lead to xenophobia, conflicts, genocide, wars.
Other meanings of the word
Term "custom" may have other meanings. In particular, this word is used to specify the institute of restriction of movement of some objects across the borders between countries. The officials, who realizes these restrictions, call themselves "custom service". (Such transformation of meaning is similar to the slang use of the word "client" among the criminal burglars and robbers; they qualify their victims as "clients")
Societies with very unexpected (for readers) and even choking customs are often described in various utopias.
The famous examples of such an utopia are travels of Gulliver by Jonatan Swift. 
flag of Tartaria
Barbarian society with broken customs is under investigation in utopia Tartaria. However, some customs of Tartaria are difficult to qualify as barbarian.
There is free election of coordinators and judges. The election fraud is qualified as heaviest crime against Tartarian state and Tartarian people.
The coordinator of each farm is considered as an employee of the people, nationals of his farm. His pre-electional program is considered as a contract. Failure of the coordinator to supply that he or she had promised is qualified as fraud.
Tartaria accept all refugees - while they try to follow the Laws of Tartaria.
In Tartaria, the citizen, the residents and even the visitors have very high freedom; it is limited only by freedom of other people. In particular, any free human can get credit risking/selling his own body and freedom.. This custom makes Tartaria dangerous for people with unstable psychics.
http://ufn.ru/tribune/trib120111 D.Kouznetsov. Place of science and physics in the human knowledge. Physics-Uspekhi, v.181, Трибуна, p.1-9 (2011, in Russian)
http://mizugadro.mydns.jp/PAPERS/2010mestoe.pdf English PDF version
- http://www.asiarooms.com/en/travel-guide/thailand/culture-of-thailand/thai-customs-and-traditions.html Thai Customs and Traditions
- http://www.greeka.com/greece-traditions.htm Greece customs and traditions
- http://samlib.ru/k/kuznecow_d_j/nihilisttaboo.shtml Nihilist: Taboos
- http://viktor-ch.livejournal.com/287453.html Виктор Шмаков. всё относительно... 2013-02-09 15:54:00.
- http://samlib.ru/k/kuznecow_d_j/practicizm.shtml Positivistic pluralism (Позитивистский плюрализм, In Russian)
- https://en.wikisource.org/wiki/Gulliver%27s_Travels Gulliver's Travels
- http://www.idph.com.br/conteudos/ebooks/BraveNewWorld.pdf Brave New World By Aldous Leonard Huxley. 18 de maio de 2002. .. The copyright on this work has since expired, and since no renewals have been found at the U.S. Copyright Office, it is in the public domain in the United States.
- https://www.e-reading.club/chapter.php/149381/8/Sheckley_-_Citizen_in_Space.html Robert Sheckley. A Ticket to Tranai.
http://voinskayachast.net/suhoputnie-voyska/specialnie/vch17845 Fiddler on the Roof. - Tradition ( with subtitles ) guru006 Published on Dec 24, 2006.