Marxism is a method of socioeconomic analysis that originates in the works of 19th century German philosophers Karl Marx and Friedrich Engels. Marxism analyzes and critiques the development of class society and especially of capitalism as well as the role of class struggles in systemic, economic, social and political change. It frames capitalism through a paradigm of exploitation and analyzes class relations and social conflict using a materialist interpretation of historical development (now known as "historical materialism") – materialist in the sense that the politics and ideas of an epoch are determined by the way in which material production is carried on.

From the late 19th century onward, Marxism has developed from Marx's original revolutionary critique of classical political economy and materialist conception of history into a comprehensive, complete world-view. There are now many different branches and schools of thought, resulting in a discord of the single definitive Marxist theory. Different Marxian schools place a greater emphasis on certain aspects of classical Marxism while rejecting or modifying other aspects. Some schools of thought have sought to combine Marxian concepts and non-Marxian concepts which has then led to contradictory conclusions.

Marxism–Leninism and its offshoots are the most well-known Marxist schools of thought as they were a driving force in international relations during most of the 20th century.

Marxism analyzes the material conditions and the economic activities required to fulfill human material needs to explain social phenomena within any given society. It assumes that the form of economic organization, or mode of production, influences all other social phenomena—including wider social relations, political institutions, legal systems, cultural systems, aesthetics, and ideologies; this serves as the fundamental axiom of Marxist thought. The economic system and these social relations form a base and superstructure. As forces of production improve, existing forms of organizing production become obsolete and hinder further progress. As Karl Marx observed: "At a certain stage of development, the material productive forces of society come into conflict with the existing relations of production or—this merely expresses the same thing in legal terms—with the property relations within the framework of which they have operated hitherto. From forms of development of the productive forces these relations turn into their fetters. Then begins an era of social revolution". These inefficiencies manifest themselves as social contradictions in society which are, in turn, fought out at the level of the class struggle.

Under the capitalist mode of production, this struggle materializes between the minority (the bourgeoisie) who own the means of production and the vast majority of the population (the proletariat) who produce goods and services. Starting with the conjectural premise that social change occurs because of the struggle between different classes within society who are under contradiction against each other, a Marxist would conclude that capitalism exploits and oppresses the proletariat, therefore capitalism will inevitably lead to a proletarian revolution. In a socialist society, private property—in the form of the means of production—would be replaced by co-operative ownership. A socialist economy would not base production on the creation of private profits, but on the criteria of satisfying human needs—that is, production would be carried out directly for use. As Friedrich Engels said: "Then the capitalist mode of appropriation in which the product enslaves first the producer, and then the appropriator, is replaced by the mode of appropriation of the product that is based upon the nature of the modern means of production; upon the one hand, direct social appropriation, as means to the maintenance and extension of production on the other, direct individual appropriation, as means of subsistence and of enjoyment".

Marxian economics and its proponents view capitalism as economically unsustainable and incapable of improving the living standards of the population due to its need to compensate for falling rates of profit by cutting employee's wages, social benefits and pursuing military aggression. The socialist system would succeed capitalism as humanity's mode of production through workers' revolution. According to Marxian crisis theory, socialism is not an inevitability, but an economic necessity.

Classical Marxism is the economic, philosophical and sociological theories expounded by Marx and Engels as contrasted with later developments in Marxism, especially Leninism and Marxism–Leninism. Orthodox Marxism is the body of Marxism thought that emerged after the death of Marx and which became the official philosophy of the socialist movement as represented in the Second International until World War I in 1914. Orthodox Marxism aims to simplify, codify and systematize Marxist method and theory by clarifying the perceived ambiguities and contradictions of classical Marxism. The philosophy of orthodox Marxism includes the understanding that material development (advances in technology in the productive forces) is the primary agent of change in the structure of society and of human social relations and that social systems and their relations (e.g. feudalism, capitalism and so on) become contradictory and inefficient as the productive forces develop, which results in some form of social revolution arising in response to the mounting contradictions. This revolutionary change is the vehicle for fundamental society-wide changes and ultimately leads to the emergence of new economic systems.

As a term, orthodox Marxism represents the methods of historical materialism and of dialectical materialism and not the normative aspects inherent to classical Marxism, without implying dogmatic adherence to the results of Marx's investigations.