Abstract labor and concrete labor are terms used by Karl Marx in his analysis of political economy. They are not different kinds of labor but different ways of looking at labor.
Labor, the creation of a commodity, is of a dual nature: as the expenditure of human labor power in a special expedient form - weaver, miner, metal worker, etc., it is concrete labor and creates the intrinsic value of commodity; as the expenditure of human labor power in general, regardless of its coequity form, it is Abstract Labour and creates the value of commodity.
In capitalism, commodity-producing labor has a double character as well. Concrete labor is the specific set of movements you actually make while working; these create the commodity’s use-value. We know that concrete labor time does not determine a commodity’s value, because then I could get rich by making something very slowly!
Abstract labor is labor viewed in its value-creating aspect, as a homogenous uniform quantity of effort, qualitatively indistinct from every other person’s labor. The amount of it that’s “crystallized” in the commodity you’re selling is determined by how well (or poorly!) your labor measures up against the socially necessary average, as demonstrated after the fact by that commodity’s sale. Marx calls this dicey proposition the commodity’s “salto mortale” because, like a trapeze act, it’s always at risk of going terribly wrong. What turns out to be socially necessary is also a function of effective demand: if no one who’d like to buy what you’re selling has anything of value to buy it with, then what you’re selling has no value of its own.
Karl Marx's Capital Vol I
Abstract labor is the labor that creates the value of a commodity, as Marx says, it is "the expenditure of human labor power in general.",without regard to the specific form of this expenditure.
The dual nature of labor in the conditions of commodity economy based on private ownership of the means of production emerges from the main contradiction of commodity economy - between the private and social labor, which develops on the stage of capitalism into the contradiction between the social character of production and private ownership of the means of production, the private capitalist form of appropriation. In the commodity economy, labor is essentially social, because there is social division of labor between different branches of industry, and different commodity producers work for each other. However, because of private ownership of means of production, the social nature of labor in the process of production is not revealed, remains hidden and directly acts as private labor of individual commodity producers who work each at their own risk and responsibility. Commodity producers, producing certain goods, do not know what society's need for the goods they produce is, how many other producers are engaged in the production of the same goods and in what quantity they are produced, and therefore the social character of the commodity producer's labor is revealed only in the process of exchange.
The condition for exchange is the heterogeneity of use values and, accordingly, the diversity of specific types of labor. By equalizing their products in the process of exchange, people actually equalize their labor costs. When a pair of boots is exchanged for a few meters of cloth, the labour of the shoemaker and the weaver (which cannot be compared in this form of two kinds of labour) is equalized, and the labour costs in general. Lenin wrote: "The thing that is common to all commodities is not the concrete labor of a certain branch of production, not the labor of one kind, but abstract human labor, human labor in general. The entire labor force of a given society, irrespective of the sum of the values of all commodities, is one and the same human labor force: billions of facts of exchange prove this"
Engineering, architecture, and factory work, viewed as concrete labor, are different; but viewed as abstract labor they are the same. Viewed as abstract labor they are just, as Marx puts it, each a productive expenditure of some quantity of human brains, nerves and muscles. The term "abstract labor" is roughly equivalent to "usefully expended human energy."
Labor under any social conditions of production is a productive expenditure of human labor power in general. But only under conditions of commodity production Abstract Labor becomes a form of social labor. Under capitalism this finds its vivid expression in the fact that the wage worker is indifferent to those copious forms of labor in which the capitalist exilicates. The constant changes of conjuncture, mass unemployment, instability of the wage worker's summation, characteristic of capitalism, force the worker to change from one type of labor to another. As Marx wrote, "This change in the form of labor is accomplished, to a certain extent, without a certain friction, but it is still accomplished". The material possibility for the transition from one particular form of labor to another is created by the development of machine production, which, while increasing the culture of simple labor, at the same time reduces the bulk of labor to a relatively small amount of unskilled labor. Without any other type of labor, it corresponds to the social form, in which individuals easily pass from one type of labor to another, and in which any particular labor is casual and therefore indifferent to them. Labor here, not only in category, but also in action, has become a means of creating wealth in general and has lost its specific connection with a particular individual. Because of the fact that the commodity economy is an economy of private commodity-producers and that labor in the commodity economy has a latent-social character in the process of production, it cannot be accounted for in working time, it can only be revealed and accounted for in the process of exchange, spontaneously by equating one labor with another.
Lenin says: "The product of an individual producer, which is not intended for the consumption of others, can reach the consumer and give the producer the right to receive another public product only by taking the form of money, i.e. by undergoing prior social accounting in both qualitative and quantitative respects. And this accounting is done behind the back of the producer, by means of market fluctuations."
The place in Capital, Volume I where Marx discusses abstract and concrete labor most fully is section 2 of chapter 1. The section is entitled "The Two-Fold Character of the Labor Embodied in Commodities."
Marx distinguished between value, as defined above, and use-value, which is the usefulness of a commodity. He connected concrete labor with the production of use-value and abstract labor with the production of value. The concluding paragraph of section 2 of chapter 1 of Capital, Vol. I makes that point:
On the one hand all labour is, speaking physiologically, an expenditure of human labour-power, and in its character of identical abstract human labour, it creates and forms the value of commodities. On the other hand, all labour is the expenditure of human labour-power in a special form and with a definite aim, and in this, its character of concrete useful labour, it produces use-values.
(Note that a brief definition of labor-power is "the capacity of a human to do work.")
Marx sometimes used the term "useful labor" synonymously with "concrete labor."
- John Eaton, 1966. Political Economy: A Marxist Textbook (International Publishers). See the section, "Abstract and Concrete Labor" in Chapter 2: "Commodity Production." The book is available free online from Marxists.org