Capital is an accumulable right of exploitation that guarantees its holder the use of a certain amount of social resources and hours of labor. Its system of circulation and accumulation ensures that those applications that contribute most to the increase in overall unpaid labor dispose of a greater proportion of resources and labor to exploit in each cycle.
The right to dispose of unpaid labor
Capital, therefore, is not only, as Adam Smith says, the command over labour. It is essentially the command over unpaid labour. All surplus-value, whatever particular form (profit, interest, or rent) it may subsequently crystallise into, is in substance the materialisation of unpaid labour-time. The secret of the self-expansion of capital resolves itself into having the disposal of a definite quantity of other people's unpaid labour.
Karl Marx. Capital, Book I, chapter XVI, 1867
As money and as commodity
Capital is therefore, above all, a social institution, a set of social relations and "rights" guaranteed to its holders. That is why, throughout its process of accumulation, it alternately takes the form of money and of commodity. It can be the money with which the commodities necessary for the production process are bought, including labor power, or the machine and the tools, material or immaterial, that are used in it.
If we fix the particular forms of appearance that self-valorizing value alternately adopts in its life cycle, we arrive at the following statements: capital is money, capital is commodities. But in reality value here becomes the subject of a process in which, while continually changing the forms of money and commodity, it alters its own magnitude, throws off surplus value from itself as original value, valorizes itself. The movement in which it adds surplus value is in fact its own movement, and its valorization is therefore self-valorization. It has acquired the occult quality of adding value because it is value. It brings forth living offspring, or at least lays golden eggs.
Karl Marx. "Capital", Book I, chapter IV, 1867
Constant and variable
The capital used in production can be broken down into two parts, both in the production process and in the process of value creation. If in the production process we have an objective component (machines, land, money...) and a subjective one, inseparable from flesh-and-blood people (labor power), in the process of valorization these components become "constant capital" and "variable capital".
The first is the part that is converted into objects useful for production, the second the part that is converted into labor power, living labor that generates value.
That part of capital then, which is represented by the means of production, by the raw material, auxiliary material and the instruments of labour, does not, in the process of production, undergo any quantitative alteration of value. I therefore call it the constant part of capital, or, more shortly, constant capital.
On the other hand, that part of capital, represented by labour-power, does, in the process of production, undergo an alteration of value. It both reproduces the equivalent of its own value, and also produces an excess, a surplus-value, which may itself vary, may be more or less according to circumstances. This part of capital is continually being transformed from a constant into a variable magnitude. I therefore call it the variable part of capital, or, shortly, variable capital. The same elements of capital which, from the point of view of the labour-process, present themselves respectively as the objective and subjective factors, as means of production and labour-power, present themselves, from the point of view of the process of creating surplus-value, as constant and variable capital.
Karl Marx. "Capital", Book I, chapter VI