Socialism is a socio-economic system in which property and the distribution of wealth are subject to control by the workers, either directly through popular collectives such as workers' councils, or indirectly exercised on behalf of the people by the state, and in which Egalitarianism or equality is an important goal. Thus, under Socialism, the means of production are owned by the state, community or the workers (as opposed to privately owned as under Capitalism).

Adherents of Socialism are split into differing, and sometimes opposing, branches, particularly between reformists and revolutionaries, and some of these are briefly describe in the Types of Socialism section below.

The term "socialism" is variously attributed to Pierre Leroux (1798 - 1871) or to Marie Roch Louis Reybaud (1799 - 1879) or to Robert Owen (1771 - 1858) in the mid-19th Century. According to Frederick Engels (1820 - 1895), by 1847, the term "socialism" (usually referring to the utopian philosophies of Robert Owen and Charles Fourier (1772 - 1837), was considered quite respectable on the continent of Europe, while "communism" was the opposite.

Certain elements of socialist thought long predate the socialist ideology that emerged in the first half of the 19th Century. For example, Plato's "The Republic" and Sir Thomas More's "Utopia", dating from 1516, have been cited as including Socialist or Communist ideas.

Modern Socialism emerged in early 19th Century Britain and France, from a diverse array of doctrines and social experiments, largely as a reaction or protest against some of the excesses of 18th and 19th Century Capitalism. Early 19th Century Socialist thought was largely utopian in nature, followed by the more pragmatic and revolutionary Socialist and Communist movements in the later 19th Century.

Social critics in the late 18th Century and early 19th Century such as Robert Owen (1771 - 1858), Charles Fourier (1772 - 1837), Pierre-Joseph Proudhon (1809 - 1865), Louis Blanc (1811 - 1882) and Henri de Saint-Simon (1760 - 1825) criticized the excesses of poverty and inequality of the Industrial Revolution, and advocated reforms such as the egalitarian distribution of wealth and the transformation of society into small utopian communities in which private property was to be abolished.

Some socialist religious movements, such as the Shakers in America, also date from this period, as does the Chartist movement for political and social reform in the United Kingdom (possibly the first mass working class movement in the world).

It was Karl Marx, though, who first employed systematic analysis (sometimes known as "scientific socialism") in an ambitious attempt to expose Capitalism's contradictions and the specific mechanisms by which it exploits and alienates. His ambitious work "Das Kapital", the first volume of which was published in 1867 with two more edited and published after his death by Friedrich Engels (1820 - 1895), is modelled to some extent on Adam Smith's "Wealth of Nations", one of the cornerstones of Capitalist theory. In it, he transforms Smith's labour theory of value into his own characteristic "law of value" (that the exchange value of a commodity is actually independent of the amount of labour required to appropriate its useful qualities), and reveals how commodity fetishism obscures the reality of Capitalist society.

In 1864, the International Workingmen's Association (IWA) or First International, was founded in London, and became the first major international forum for the promulgation of Socialist ideas, under the leadership of Marx and Johann Georg Eccarius. Anarchists, like the Russian Mikhail Bakunin (1814 - 1876), and proponents of other alternative visions of Socialism which emphasized the potential of small-scale communities and agrarianism, coexisted with the more influential currents of Marxism and social democracy. Much of the developement of Socialism is indistinguishable for the development of Communism, which is essentially an extreme variant of Socialism.

Marx and Engels, who together had founded the Social Democratic Workers' Party of Germany in 1869, were also responsible for setting up the Second International (or Socialist International) in 1889, as the ideas of Socialism gained new adherents, especially in Central Europe, and just before his death in 1895, Engels boasted of a "single great international army of socialists".

When the First World War started in 1914, the socialist social democratic parties in the UK, France, Belgium and Germany supported their respective states' war effort, discarding their commitment to internationalism and solidarity, and the Second International dissolved during the war.

In Russia, however, Vladimir Ilyich Lenin (1870 - 1924) denounced the war as an imperialist conflict, and urged workers worldwide to use it as an occasion for proletarian revolution. In February 1917, revolution broke out in Russia and the workers, soldiers and peasants set up councils (or soviets in Russian). The Bolsheviks won a majority in the soviets in October 1917 and, at the same time, the October Revolution was led by Lenin and Leon Trotsky (1879 - 1940). The new Soviet government immediately nationalized the banks and major industries, repudiated the former Romanov regime's national debts, sued for peace and withdrew from the First World War, and implemented a system of government through the elected workers' councils or soviets. The Third International (also known as the Communist International or Comintern) was an international Communist organization founded in Moscow in 1919 to replace the disbanded Second International.

After Lenin's death in 1924, the Communist Party of the Soviet Union, under Josef Stalin declared a policy of "socialism in one country", taking the route of isolationism. This led to a polarization of Socialism around the question of the Soviet Union and adoption of socialist or social democratic policies in response, or in other cases the vehement repudiation of all that it stands for.

However, not everyone saw Socialism as necessarily entailing revolution, and non-revolutionaries such as the influential economists John Maynard Keynes (1883 - 1946) and John Kenneth Galbraith (1908 - 2006), took inspiration from the work of John Stuart Mill as well as Marx, and provided theoretical justification for (potentially very extensive) state involvement in an existing market economy. This kind of Social Democracy (and the more left-wing Democratic Socialism) can be considered a moderate form of Socialism (although many socialists would not), and aims to reform Capitalism democratically through state regulation and the creation of state-sponsored programs and organizations which work to ameliorate or remove injustices purportedly inflicted by the Capitalist market system.

Criticisms of Socialism range from disagreements over the efficiency of socialist economic and political models, to outright condemnation of socialist states.

Some critics dispute that the egalitarian distribution of wealth and the nationalization of industries advocated by some socialists can be achieved without loss of political or economic freedoms. Some argue that countries where the means of production are socialized are less prosperous than those where the means of production are under private control. Yet others argue that socialist policies reduce work incentives (because workers do not receive rewards for a work well done) and reduce efficiency through the elimination of the profit and loss mechanism and a free price system and reliance on central planning. They also argue that Socialism stagnates technology due to competition being stifled. The tragedy of the commons effect has been attributed to Socialism by some, whereby when assets are owned in common, there are no incentives in place to encourage wise stewardship (i.e. if everyone owns an asset, people act as if no-one owns it). There has also been much focus on the economic performance and human rights records of Communist states, although this is not necessarily a criticism of Socialism.

Socialists have counter-argued that Socialism can actually increase efficiency and economic growth better than Capitalism, or that a certain degree of efficiency can and should be sacrificed for the sake of economic equality or other social goals. They further argue that market systems have a natural tendency toward monopoly or oligopoly in major industries, leading to a distortion of prices, and that a public monopoly is better than a private one. Also, they claim that a socialist approach can mitigate the role of externalities in pricing. Some socialists have made a case for Socialism and central planning being better able to address the issue of managing the environment than self-serving Capitalism.

• Democratic Socialism advocates Socialism as an economic principle (the means of production should be in the hands of ordinary working people), and democracy as a governing principle (political power should be in the hands of the people democratically through a co-operative commonwealth or republic). It attempts to bring about Socialism through peaceful democratic means as opposed to violent insurrection, and represents the reformist tradition of Socialism.
  It is similar, but not necessarily identical (although the two terms are sometimes used interchangeably), to Social Democracy. This refers to an ideology that is more centrist and supports a broadly Capitalist system, with some social reforms (such as the welfare state), intended to make it more equitable and humane. Democratic Socialism, by contrast, implies an ideology that is more left-wing and supportive of a fully socialist system, established either by gradually reforming Capitalism from within, or by some form of revolutionary transformation.

• Revolutionary Socialism advocates the need for fundamental social change through revolution or insurrection (rather than gradual refom) as a strategy to achieve a socialist society. The Third International, which was founded following the Russian Revolution of 1917, defined itself in terms of Revolutionary Socialism but also became widely identified with Communism. Trotskyism is the theory of Revolutionary Socialism as advocated by Leon Trotsky (1879 - 1940), declaring the need for an international proletarian revolution (rather than Stalin's "socialism in one country") and unwavering support for a true dictatorship of the proletariat based on democratic principles. Luxemburgism is another Revolutionary Socialist tradition, based on the writings of Rosa Luxemburg (1970 - 1919). It is similar to Trotskyism in its opposition to the Totalitarianism of Stalin, while simultaneously avoiding the reformist politics of modern Social Democracy.

• Utopian Socialism is a term used to define the first currents of modern socialist thought in the first quarter of the 19th Century. In general, it was used by later socialist thinkers to describe early socialist, or quasi-socialist, intellectuals who created hypothetical visions of perfect egalitarian and communalist societies without actually concerning themselves with the manner in which these societies could be created or sustained. They rejected all political (and especially all revolutionary) action, and wished to attain their ends by peaceful means and small experiments, which more practical socialists like Karl Marx saw as necessarily doomed to failure. But the early theoretical work of people like Robert Owen (1771-1858), Charles Fourier (1772-1837) and Étienne Cabet (1788–1856) gave much of the impetus to later socialist movements.

• Libertarian Socialism aims to create a society without political, economic or social hierarchies, in which every person would have free, equal access to tools of information and production. This would be achieved through the abolition of authoritarian institutions and private property, so that direct control of the means of production and resources will be gained by the working class and society as a whole. Most Libertarian Socialists advocate abolishing the state altogether, in much the same way as Utopian Socialists and many varieties of Anarchism (including Social Anarchism, Anarcho-Communism, Anarcho-Collectivism and Anarcho-Syndicalism).

• Market Socialism is a term used to define an economic system in which there is a market economy directed and guided by socialist planners, and where prices would be set through trial and error (making adjustments as shortages and surpluses occur) rather than relying on a free price mechanism. By contrast, a Socialist Market Economy, such as that practiced in the People's Republic of China, in one where major industries are owned by state entities, but compete with each other within a pricing system set by the market and the state does not routinely intervene in the setting of prices.

• Eco-Socialism (or Green Socialism or Socialist Ecology) is an ideology merging aspects of Marxism, Socialism, Green politics, ecology and the anti-globalization movement. They advocate the non-violent dismantling of Capitalism and the State, focusing on collective ownership of the means of production, in order to mitigate the social exclusion, poverty and environmental degradation brought about (as they see it) by the capitalist system, globalization and imperialism.

• Christian Socialism generally refers to those on the Christian left whose politics are both Christian and socialist, and who see these two things as being interconnected. Christian socialists draw parallels between what some have characterized as the egalitarian and anti-establishment message of Jesus, and the messages of modern Socialism.