Law and Chaos are cosmic forces engaged in an ageless, eternal struggle of civilization (Law) against the powers of blood-drenched anarchy and dissolution (Chaos). This is the ultimate battleground of demons and godlings, unspeakable ancient horrors and distant powers of divinity, fought across all the planes of existence and in all the nuances of the profound and incomprehensible multiverse. There are cosmic forces of Neutrality as well, standing aloof from the battle between Law and Chaos and working toward their own inscrutable ends. This is the realm of matters that humankind is not meant to know; those who delve too deeply into blasphemous lore and higher realities do so at the very real risk of their sanity.
This vast opposition extends into every corner of the universe; great and horrible demon-princes forge their subtle plans to bring ruination to all things, while the hosts of Law marshal their own allies against ancient foes and new, rising, threats to civilization. Even the player Characters are involved with this eternal struggle, whether directly or indirectly, for every action tilts the balance infinitesimally to one side or the other. Will the adventurers attempt to stay out of such matters, remaining neutral between the opposing forces? Will they become champions of Law and civilization, heroes against the mad desolation of Chaos? Or will they themselves fall into the subtle lure of power offered by the legions of anarchy?
All Characters are aligned with Law, Chaos, or Neutrality. Any person who is active on behalf of civilization and goodness is considered to be Lawful, while one who has fallen into the dark realms of demonic beliefs or vicious cruelty is considered Chaotic. Characters who fall into the grey areas of morality in between these extremes are considered Neutral. Druids, who sanctify the forces of nature and its holy places, are a good example of Neutrality since they support neither civilization nor anarchy. Most Thieves, too, are neutral, for they follow a course of self-interest without (usually) being bloody-handed or homicidal. Virtually all monsters are, by their very nature, servants of Chaos.
It is not necessary, of course, to run a “heroic” game of Swords & Wizardry; many adventuring parties are out for fame and fortune, and do not want to get tangled up in the opposition of Law and Chaos. These freebooters might have their own codes of personal morality, but heroism and honor seldom guide their actions if balanced against survival and loot.