There are three classes to our society: lower, middle, and upper. The lower class is comprised of individuals who earn their living with their hands. The middle class is made up of those who earn their living with their brains. The upper class differs from these two in that they don't "earn" their living at all—they "make" their living.
The upper class is comprised of builders in the general sense. They collectively compose and execute the major changes in civilization. Their wealth is not in bank accounts, but in assets they have built. Sometimes they fund their own ventures, and sometimes they partner with investors, but their aim is to produce value.
The middle class came out of industrialization—particularly the advent of the assembly line, which needed not only laborers but also specialists and managers to oversee the operations. They work for the owners or operate a "small" business which is an extension of their individual skill and expertise.
The lower class are also ultimately the consumers of everything produced by society. Since their work does not extend beyond their physical abilities, they rarely create value greater that what they consume. They work for the owners, and also work under the managers. By definition, they create nothing and merely operate in a reactionary mode—following orders. If they ever become proactive, they cease to be lower class.
In the early days, the middle class was not far behind the upper class. However, this may have had less to do with the height of the upper middle class and more with underdevelopment of the upper class's assets. As industrialization matured and organizations grew larger, the divide between the middle and upper became more apparent.
The system is such that the middle class's value creating ability is limited to what they personally can do. In the beginning, it would have seemed like a better deal to work a middle class job for what was a very good salary, and leave the change-making and ownership responsibilities to those who would have made barely any more for their efforts.
However, because the middle class is dependent upon the application of their personal skills, they are limited in income by the amount of time they have to ply their trade. By contrast, since the owners' wealth is not dependent upon an application of their skills—except what's needed to induce growth—the sky is the limit. As long as they increase the value of their brand, company, or organization, they increase their net-worth.
So the only thing stopping the lower classes from performing the effort to attain higher class status, is information. Proper education from the right sources—particularly, sources demonstrating the results you want in life—is then essential to rising above economic despair.