d ) Capital
Capital is a production relation which establishes a completely new and extremely efficient bond between living labour and past labour ( accumulated by previous generations ). But as with the birth of exchange, the rise of capital is not the result of a decision or a plan, but a consequence of real social relations which lead to a qualitatively new development in certain Western European countries after the Middle Ages.
Merchants had accumulated large sums of money, in various forms, and perfected systems of banking and credit. It was possible to use these sums : the first machines ( textile ) were invented, and thousands of poor people ( former peasants or craftsmen ) had lost their instruments of production and were forced to accept the new production relation : wage-labour. The prerequisite was accumulated, stored up labour in the form of machines ( and later, factories ). This past labour was to be set in motion by the living labour of those who had not been able to realise such an accumulation of raw materials and means of production. This was the foundation on which capital was established. From the dissolution of the primitive community to the end of the Middle Ages ( in Western Europe; the evolution was different elsewhere ), there was an exchange of goods produced by slaves, craftsmen, free peasants, and some wage workers, depending on the period. Around the 15th century only the surpluses of small peasant production and a few manufactured goods ( weapons, clothes ) were really objects of trade. Exchange was neither the aim nor the regulator of production. Commerce alone, simple commodity production ( as opposed to capitalist commodity production ) could not provide the stability, the durability required by the socialisation and unification of the world. This was accomplished by capitalist commodity production, and the means with which it accomplished this consisted of the production which it took over. Indeed, capital realised an actual synthesis of exchange and production. 
The slave did not sell his labour power : his owner bought the slave himself, and put him to work. In capitalism, living labour is bought by the means of production which it sets into motion. The role of the capitalist is not negligible, but quite secondary : "the capitalist as such is only a function of capital,"  the leader of social production. What is important is the development of past labour by living labour. To invest, to accumulate -- these are the mottoes of capital ( the priority given to heavy industry in all the so-called socialist countries is nothing other than the sign of the development of capitalism ). But the aim of capital is not to accumulate use values. Capital only multiplies factories, railways, etc., to accumulate value. Capital is first of all a sum of value, of abstract labour crystallised in the form of money, finance capital, shares, bonds, etc., which tries to increase. A sum x of value must give x + profit at the end of the cycle. To valorise itself, value buys labour power. The great change introduced by capital is to turn labour power into a commodity.
This commodity is quite special, as its consumption furnishes work, hence new value; whereas means of production yield no more than their own value. Therefore the use of labour power furnishes a supplementary value. The origin of bourgeois wealth is to be found in this surplus value, in this difference between the value created by the wage-labourer in his work, and the value necessary for the reproduction of his labour-power. Wages only cover the expenses of that reproduction ( the means of subsistence of the worker and his family ). Working time is divided ( not consciously, but in fact ) into two parts : one part is devoted to the reproduction of the labour power, and is paid by wages; the other part is devoted to the production of new value, and is unpaid labour.
It is easy to see from this analysis that the essential fact is not the appropriation of surplus-value by the capitalist as an individual. Communism has nothing to do with the idea that workers have to partially or totally recover the surplus value for themselves,  for a simple and obvious reason : some of the resources must be used for the renewal of equipment, for new production, etc. The point is not that a handful of people take a disproportionately large share of the surplus-value. If these people were eliminated, while the rest of the system remained the same, part of the surplus-value would be given to the workers and the rest would be invested in collective and social equipment, welfare, etc. : this is in fact the program of the left, including the official C.P.s. Actually the logic of the system of value would always result in the development of production for a maximal valorisation. As long as the basis of society is a mechanism mingling two processes, a process of real work, and a process of valorisation, value dominates society. The change brought about by capital is to have conquered production, and thus to have socialised the world since the 19th century, with industrial plants, means of transportation, storage, and quick transmission of information. But in the capitalist cycle the fulfilment of needs is only a by-product, and not the driving force of the mechanism. Valorisation is the aim : fulfilment of needs is at best a means, since what has been produced must be sold.
The enterprise is the location and the centre of capitalist production; each industrial or agricultural enterprise works as a rallying point for a sum of value looking for an increase. The enterprise must make profits. Here again the law of profit has nothing to do with the action of a few "big" capitalists, and communism does not mean getting rid of "big" capitalists. What matters is not the individual profits made by capitalists, but the constraint, the orientation imposed upon production and society by this system which dictates how to work and what to consume. The whole demagogy about the rich and the poor confuses the issue. Communism does not mean taking money from the rich, nor revolutionaries distributing it to the poor.
 Marx, Capital, Volume I, Part ll : "The Transformation of Money into Capital."
 Marx, Theories of Surplus-Value, Part I ( Lawrence and Wishart, 1969 ), p. 409.
 One can find many interesting ideas in the ( half-mad ) text published by the International Communist Party : The Fundamentals of Communism ( Paris, 1972 ).