We have had a number of discussions recently (for example those on the Pope's recent statements) where it is clear that there is no shared definition of 'capitalism'. This is, perhaps, not surprising, since (a) it is notoriously difficult to define; and (b) many of the definitions (and the term itself) come from capitalism's enemies.
A fairly standard dictionary definition (Merriam-Webster) is:
an economic system characterized by private or corporate ownership of capital goods, by investments that are determined by private decision, and by prices, production, and the distribution of goods that are determined mainly by competition in a free market
Some take exception to this approach. For them the essential definition is:
Capitalism is a social system based on the principle of individual rights.
(And, in particular, property rights). This captures something important, but seems to miss out too much.
One Catholic approach is that of Pius XI in Quadragesimo Anno (@100):
that economic system, wherein, generally, some provide capital while others provide labor for a joint economic activity.
There doesn't seem to be much to object to in the system described by any of these definitions, so the critics of capitalism must have something else in mind. To stick with the (or, at least, a) Catholic critique:
A capitalist society will almost inevitably turn into a commercial society, a society in which mere accumulation of wealth is sought, in which almost any legal method of acquiring wealth is looked upon with approval, and, most banefully, in which striving for wealth is seen as the primary occupation of mankind. The “love of money” which St. Paul excoriates now replaces love of God, love of family and friends, love of learning as society’s chief pursuit. ... It is true that in a capitalist society not all individuals consider mere accumulation of money or things as the purpose of life. But a capitalist society tends to disregard other motives as unworthy of mention. Education, for example, is justified in terms of how much it will increase one’s salary, and the institutionalized pressure for ever greater sales, ever improving performance over the previous quarter, pushes many into making mere gain their goal. Capitalism hardly acknowledges the existence of any other motive or reason for being except accumulation of wealth.
That is, it is capitalism's moral (spiritual) results that are decried, rather than capitalism itself. (One might wonder at the historical or sociological accuracy of that "almost inevitably" in the first line of the quote...)
Marx's critique is encyclopedic, incisive, confused and, at times, silly. Capitalism is unjust, because it appropriates value created by the laborer (in favor of capital). It is is unjust, because, logically, it requires the progressive immiseration of the working class. It is doomed, because by its internal logic (or, rather, contradictions) it gives rise to periodic and ever greater crises in its own system. It is contrary to nature, because it alienates the worker from his labor (and other things), and thus from his humanity.
What does 'capitalism' mean to you? (I had thought of it as what happens when a free people with property rights and certain other social institutions are allowed to get on with their lives.) Are you a 'capitalist'? (I would have said I am a skeptic with respect to the efficacy and justice of alternative systems, and other attempts to interfere in the freely-chosen relations of human beings.)