For Karl Marx capitalism was a mode of production that was based on the "private ownership of the means of production". This means that the capitalists (owners of factories, materials, machines, in other words, the means of production), need to employ workers which don't possess anything but their labor. These workers sell their labor in exchange of wages, in the form of a commodity, making them interchangeable.
In this economic system the companies, rahter than the state, control the trade and industry.