Marxism, true to it's name, often refers to the thoughts and theories of Karl Marx and his collaborator, Friedrich Engels. In a sense, it may refer to diverse thoughts, theories, and principles based on Marxist theory. 
The fundamental ideology of communism, Marxism holds that all people are entitled to enjoy the fruits of their labour but are prevented from doing so in a capitalist economic system, which divides society into two classes: nonowning workers and nonworking owners. Marx called the resulting situation “alienation,” and he said that when the workers repossessed the fruits of their labour, alienation would be overcome and class divisions would cease.
Labor Theory of Value
The labor theory of value is a major pillar of traditional Marxian economics, which is evident in Marx’s masterpiece, Capital (1867). The theory’s basic claim is simple: the value of a commodity can be objectively measured by the average number of labor hours required to produce that commodity.
If a pair of shoes usually takes twice as long to produce as a pair of pants, for example, then shoes are twice as valuable as pants. In the long run, the competitive price of shoes will be twice the price of pants, regardless of the value of the physical inputs.
Although the labor theory of value is demonstrably false, it prevailed among classical economists through the midnineteenth century. Adam Smith, for instance, flirted with a labor theory of value in his classic defense of capitalism, The Wealth of Nations (1776), and David Ricardo later systematized it in his Principles of Political Economy (1817), a text studied by generations of free-market economists.
So the labor theory of value was not unique to Marxism. Marx did attempt, however, to turn the theory against the champions of capitalism, pushing the theory in a direction that most classical economists hesitated to follow. Marx argued that the theory could explain the value of all commodities, including the commodity that workers sell to capitalists for a wage. Marx called this commodity “labor power.”
Labor power, Marx explains was the workers' capacity to produce goods and services. Marx, using principles of classical economics, explained that the value of labor power must depend on the number of labor hours it takes society, on average, to feed, clothe, and shelter a worker so that he or she has the capacity to work. In other words, the long-run wage workers receive will depend on the number of labor hours it takes to produce a person who is fit for work. Suppose five hours of labor are needed to feed, clothe, and protect a worker each day so that the worker is fit for work the following morning. If one labor hour equaled one dollar, the correct wage would be five dollars per day.
Marx then asked an apparently devastating question: if all goods and services in a capitalist society tend to be sold at prices (and wages) that reflect their true value (measured by labor hours), how can it be that capitalists enjoy profits—even if only in the short run? How do capitalists manage to squeeze out a residual between total revenue and total costs?
Capitalists, Marx answered, must enjoy a privileged and powerful position as owners of the means of production and are therefore able to ruthlessly exploit workers. Although the capitalist pays workers the correct wage, somehow—Marx was terribly vague here—the capitalist makes workers work more hours than are needed to create the worker’s labor power. If the capitalist pays each worker five dollars per day, he can require workers to work, say, twelve hours per day—a not uncommon workday during Marx’s time. Hence, if one labor hour equals one dollar, workers produce twelve dollars’ worth of products for the capitalist but are paid only five. The bottom line: capitalists extract “surplus value” from the workers and enjoy monetary profits.
Although Marx tried to use the labor theory of value against capitalism by stretching it to its limits, he unintentionally demonstrated the weakness of the theory’s logic and underlying assumptions. Marx was correct when he claimed that classical economists failed to adequately explain capitalist profits. But Marx failed as well. By the late nineteenth century, the economics profession rejected the labor theory of value. Mainstream economists now believe that capitalists do not earn profits by exploiting workers (see profits). Instead, they believe, entrepreneurial capitalists earn profits by forgoing current consumption, by taking risks, and by organizing production.
Theory of History
Marx did not set out his theory of history in great detail. Accordingly, it has to be constructed from a variety of texts, both those where he attempts to apply a theoretical analysis to past and future historical events, and those of a more purely theoretical nature. Of the latter, the 1859 Preface to A Critique of Political Economy has achieved canonical status. However, The German Ideology, co-written with Engels in 1845, is a vital early source in which Marx first sets out the basics of the outlook of historical materialism. We shall briefly outline both texts, and then look at the reconstruction of Marx's theory of history in the hands of his philosophically most influential recent exponent, G.A. Cohen, who builds on the interpretation of the early Russian Marxist Plekhanov.
We should, however, be aware that Cohen's interpretation is not universally accepted. Cohen provided his reconstruction of Marx partly because he was frustrated with the existing Hegelian-inspired 'dialectical' interpretations of Marx associated especially with Louis Althusser, which he felt did not provide a rigorous account of Marx's views. However, some scholars believe that the interpretation that we shall focus on is faulty precisely for its lack of attention to the dialectic. One aspect of this criticism is that Cohen's understanding has a surprisingly small role for the concept of class struggle, which is often felt to be central to Marx's theory of history. Cohen's explanation for this is that the 1859 Preface, on which his interpretation is based, does not give a prominent role to class struggle, and indeed it is not explicitly mentioned. Yet this reasoning is problematic for it is possible that Marx did not want to write in a manner that would engage the concerns of the police censor, and, indeed, a reader aware of the context may be able to detect an implicit reference to class struggle through the inclusion of such phrases as “then begins an era of social revolution,” and “the ideological forms in which men become conscious of this conflict and fight it out”. Hence it does not follow that Marx himself thought that the concept of class struggle was relatively unimportant. Furthermore, when A Critique of Political Economy was replaced by Capital, Marx made no attempt to keep the 1859 Preface in print, and its content is reproduced just as a very much abridged footnote in Capital. Nevertheless we shall concentrate here on Cohen's interpretation as no other account has been set out with comparable rigour, precision and detail.
- http://www.econlib.org/library/YPDBooks/Marx/mrxCpA20.html#Part VI, Chapter 20