The discovery of gold and silver in America, the extirpation, enslavement and entombment in mines of the aboriginal population, the beginning of the conquest and looting of the East Indies, the turning of Africa into a warren for the commercial hunting of black-skins, signalised the rosy dawn of the era of capitalist production. These idyllic proceedings are the chief momenta of primitive accumulation. On their heels treads the commercial war of the European nations, with the globe for a theatre. It begins with the revolt of the Netherlands from Spain, assumes giant dimensions in England’s Anti-Jacobin War, and is still going on in the opium wars against China.
— Karl Marx, Capital, Chapter 31: Genesis of the Industrial Capitalist
In Marxist economics, the phenomenon of primitive accumulation (also called previous accumulation, original accumulation) of capital concerns the origin of capital, and therefore of how class distinctions between possessors and non-possessors came to be.
Adam Smith's hypothesis for primitive accumulation depicted a peaceful process, in which some workers laboured more diligently than others and gradually built up wealth, eventually leaving the less diligent workers to accept living wages for their labour. Marx rejected this explanation as "childishness", instead stating that, in the words of David Harvey, primitive accumulation "entailed taking land, say, enclosing it, and expelling a resident population to create a landless proletariat, and then releasing the land into the privatised mainstream of capital accumulation". This was accomplished through violence, war, enslavement, and colonialism.
Marxist distinction with other economists
For political economists like Smith, economy has transhistorical laws of motion (in his words, man has a natural "propensity to truck, barter, and exchange" and this is how capitalism arose). In other words, no matter the system, no matter the time, man will always have this propensity.
For Marx, however, every economic epoch has its own historically specific laws of motion. Capitalism is for Marx by no means the result of human nature but that of a historical process.
In Capital, Marx writes: "The essential difference between the various economic forms of society, between, for instance, a society based on slave-labour, and one based on wage-labour, lies only in the mode in which this surplus-labour is in each case extracted from the actual producer, the labourer."
So what distinguishes one epoch from another is the mode in which the surplus labour, that is, the total product of labour minus the amount consumed for the producers' own reproduction, is extracted. As capitalism rests on the commodification of labour power, it presupposes the appropriate production relations that make this possible. The concept of primitive accumulation for Marx refers exactly to the genesis of such production relations. In his account of the primitive accumulation in Capital, Marx traces primitive accumulation to the process of the expropriation of the peasants from their means of production in the English countryside. As peasants came to lose their access to the means of production which had been secured under feudalism in the form of customary tenures, they were increasingly forced to sell the only thing they were left to sell: their labour power.
Examples
Enclosure of the commons
A very appropriate example is the enclosure of the commons that took place in the United Kingdom. Under feudalism, British peasants enjoyed common land, which belonged to all people living in a locality and which they were able to freely use -- as grazing pastures, a source of wood, for foraging and growing crops, etc.
As the bourgeoisie became stronger in England and was able to change laws, Enclosure acts were proclaimed in the United Kingdom in the 16th and 17th century, which put this land for sale. Prices were prohibitive to peasants and only the bourgeoisie (as well as feudal lords partaking in capitalist ventures) were able to purchase it. They then closed the many hectares of land in parcels with fences or hedges and employed locals to work on this land in an effectively capitalist relation of production (the worker surrendered the product of their labour to the landowner), whereas they had for centuries used this land freely without having to pay for it or surrender their labour.