It’s a historical mode of production and therefore not eternal, made unique by relations of production based on the commercialization of labour-power. As a consequence, it’s the ultimate historic expression of the market economy, that’s to say, of an economy based on trade- the exchange of one property for another- whose premise is the private appropriation of the means of production.
The above mentioned commercialization derives from this private appropriation, from which the two classes specific to capitalism are based: property owners who buy labour-power in order to make a profit from this purchase- which is their means of subsistence- due to the fact that they only pay for the equivalent of a fraction of all of the work done; and wage workers who sell their labour-power in order to get the income that allows them to subsist. All of this is rigorously explained by Marx in Capital, which uses the law of value to explain how goods are exchanged- where their value comes from and how it’s validated or not or partially- to explain exploitation, and which culminates in the law of the tendency of the rate of profit to fall, the foundation not of capitalism’s immediately contradictory character but rather of its increasingly contradictory character and, therefore, it’s historic limits.
By definition, as in any classist society, capitalism has always been tremendously violent; one of the largest expressions of this is unemployment, i.e. the negation of large portions of the population to obtain their means of subsistence through work. For this very reason, capitalism is incompatible with democracy, given that, among other things, the decision of who works and who is denied the opportunity to work is made by a very small part of the population, the capitalists.
If it really is true that capitalism made possible an enormous development of the productive forces of production- obviously, in any case not in an ideal way- once it reached its imperialist stage at the start of the twentieth-century, it not only doesn’t allow for new systematic developments, but also increasingly systematizes its own destruction. This reveals its already completely anachronistic character.