Who is Karl Marx and what is his economic theory?

Karl Marx was a German philosopher, economist, sociologist, historian, political theorist, and revolutionary whose work in developing the theory of historical materialism and critiquing capitalism has had a significant impact on the world. His economic theory, often referred to as Marxist economics, is a materialist conception of history that posits the mode of production of material goods and the social relations that individuals enter into within a society are the basis for understanding the development of that society.
Marx's economic theory is rooted in the belief that the economic base of society – the way in which humans produce and reproduce the material conditions of life – determines the superstructure, which includes law, politics, religion, and other social institutions. He argued that history is driven by class struggle, where the interests of the ruling class (those who own the means of production) are in conflict with the working class (those who sell their labor for wages).
One of the central tenets of Marx's economic theory is the concept of surplus value. According to Marx, the value of a commodity is determined by the socially necessary labor time required to produce it. However, workers are typically paid less than the value of the goods they produce. The difference between the value of the goods and the wages paid to the workers is the surplus value, which is appropriated by the capitalist class. Marx saw this as a form of exploitation, as the workers are not fully compensated for their labor.
Marx also introduced the idea of the reserve army of labor, which refers to the unemployed and underemployed workers who are available to be drawn into the labor market when needed. This reserve army helps to keep wages low and ensures that there is a constant supply of cheap labor for capitalists to exploit.
Another key concept in Marx's economic theory is the law of value, which states that the value of a commodity is determined by the amount of socially necessary labor time required to produce it. This law, according to Marx, governs the exchange of commodities in a capitalist society. However, Marx also recognized that the actual prices of commodities can deviate from their values due to various factors such as supply and demand, competition, and the influence of the market.
Marx's critique of capitalism extends beyond the economic sphere. He argued that capitalism is not only an economic system but also a social system that shapes human consciousness and relationships. He believed that capitalism creates a society divided into two main classes – the bourgeoisie (capitalists) and the proletariat (workers) – and that this division leads to alienation, where individuals are estranged from their work, from the products of their labor, from their fellow workers, and from their own human potential.
Marx's economic theory has been influential in shaping socialist and communist movements around the world. It has also been the subject of much debate and criticism, with some arguing that it is outdated or that it has been misapplied in practice. Nevertheless, Marx's ideas continue to be studied and discussed today, particularly in the context of global economic inequality and the ongoing struggle for social justice.

Marxist economics comes from the ideas of a philosopher named Karl Marx applied to economics. He created it to explain the "rules of motion" of production and exchange under capitalism. This theory was used to argue against the middle class theories of economics common at that time.