The Fremen are the inhabitants of Arrakis, also known as Dune, the desert planet that is the sole source of the spice melange in the known universe. The Fremen came to Dune thousands of years earlier as the Zensunni Wanderers, a religious sect (as their name suggests, they are derived from Zen Buddhism and Sunni Muslims) in retreat; over time, surviving the incredibly harsh conditions of Dune bred them into the ultimate example of humans in extremis: a people bred purely to survive. They took to calling themselves the Free Men of Dune, which was later shortened to Fremen.
The Fremen are organized into sietches. Each sietch has a naib, whose word is law unless someone is willing to challenge him for leadership. The Fremen practice polygamy, apparently as a means of pinpointing male infertility. Each sietch has a sayyadina, a wise woman who serves as the acolyte of a Fremen Reverend Mother, comparable to a Bene Gesserit Reverend Mother .
The Fremen system of justice always relies ultimately on trial by combat. The naib of the tribe is someone who killed the previous naib in single combat. Anyone may challenge another in a duel to the death over matters of etiquette, law, or honour; the winner of the duel is responsible for the wife, children, and certain possessions of the loser, as well as the right of the circumstances leading to the duel. Because a duel is fought without the water-retaining stillsuit, the victor is entitled to a percentage of the deathstill-reclaimed water, to make up for the moisture sacrificed in the fight.
Fremen are some of the best hand to hand combatants in the universe. Their difficult upbringing and spartan existence ensure that only the strongest survive on Dune. Hand to hand combat has re-entered human conflict because of some very fundamental developments in technology. The personal body shield has rendered all forms of projectile weapons semi-obsolete. Energy weapons -"lasguns"-- react violently with a shield, creating an explosion comparable to sub-atomic fusion, killing operator and shield wearer. Body shielding can be compromised by moving an edged weapon at a specific speed slow enough to penetrate the tuning of a shield. Shields are known to drive the fiercely territorial sandworms of Arrakis into a killing frenzy. They are not used on Arrakis's open regions with any frequency. Consequently, Fremen have an edge because their instincts do not compensate for the shield. Fremen use different archaic weapons to great effect (firearms, crossbows), but the most deadly and prized possession of a Fremen warrior is the crysknife- a personally tuned blade ground from the tooth of a sandworm. An untreated crysknife will disintegrate soon upon the death of its owner.
The most notable custom of the Fremen is their water conservation. Living in the desert with no natural sources of water has spurred the Fremen to build their society around the collection, storage, and conservative use of water.
Water is collected from the atmosphere in windtraps that condense the humidity and add it to the underground water store. Water can also be collected from dead animals and people (especially outside wanderers) and processed in a deathstill which removes the water from the carcass for addition to the sietch water store. The Fremen that caused or discovered the death of the animal or person is then given a set of water rings whose markings denote a volume of water equal to the amount of water collected.
These rings are used as a form of currency, and are backed by fixed volumes of water (analogous to the historical gold standard). They also have a profound significance in matters of birth, death and courtship ritual.
Each sietch has its own water store underground. This store can hold millions of decaliters of water and is accounted for literally to the last drop. This store is used as a bank for all the water owned by members of the sietch through water rings, as well as for the sietch's own store of water for the eventual transformation of their planet into something other than desert.
The Fremen spend all time out of their sietch in a stillsuit, a special body-enclosing suit designed to collect and recycle all the moisture the body releases, from urine, feces and sweat, to the exhalation of water vapor in the breath.
The special fabric is a micro-sandwich designed to dissipate heat and filter wastes while reclaiming moisture. The water is then held in catchpockets and made available to drink through a tube.
A Fremen in a well-kept suit can survive weeks in the desert without any other source of water.
Because of their culture's focus on water conservation, it is generally considered a great sign of respect (though often interpreted otherwise) for a Fremen to spit before a person.