Anarchism is generally defined as the political philosophy which holds the state to be undesirable, unnecessary, and harmful, or alternatively as opposing authority in the conduct of human relations. Proponents of anarchism (known as "anarchists") advocate stateless societies based on non-hierarchical voluntary associations.
There are many types and traditions of anarchism, not all of which are mutually exclusive. Anarchist schools of thought can differ fundamentally, supporting anything from extreme individualism to complete collectivism. Strains of anarchism have been divided into the categories of social and individualist anarchism or similar dual classifications. Anarchism is often considered to be a radical left-wing ideology, and much of anarchist economics and anarchist legal philosophy reflect anti-statist interpretations of communism, collectivism, syndicalism or participatory economics. However, anarchism has always included an individualist strain supporting a market economy and private property, or morally unrestrained egoism. Some individualist anarchists are also socialists or communists while some anarcho-communists are also individualists or egoists while some anarcho-communists are also individualists. Still other self described anarchists are seen by some as far righ-wingers, such as Market Anarchists or Anarcho-capitalists, who believe in a completeley voluntary individualist society, including private property.
Anarchism as a social movement has regularly endured fluctuations in popularity. The central tendency of anarchism as a mass social movement has been represented by anarcho-communism and anarcho-syndicalism, with individualist anarchism being primarily a literary phenomenon which nevertheless did have an impact on the bigger currents and individualists also participated in large anarchist organizations. Most anarchists oppose all forms of aggression, supporting self-defense or non-violence (anarcho-pacifism), while others have supported the use of some coercive measures, including violent revolution and propaganda of the deed, on the path to an anarchist society.
Source: Wikipedia, "Anarchism", available under the CC-BY-SA License.