AKA a Lorenzen curve
The Lorenz curve is a measure of a population's income inequality developed by Max O. Lorenz in 1905. In 1912 Corrado Gini would modify it to create the Gini Coefficient, which is currently the most commonly used indicator of economic inequality.
To find a population's Lorenz curve you construct a graph in which one axis represents a percentage of the population (from 0% up to 100%) and the other represents percent of income. In a perfectly equal system, increasing the percent of the population you were looking at would result in an exactly equal increase in the the amount of income earned; 10% of the population would earn 10% of the income, 50% of the population would earn 50% of the income -- resulting in a 'curve' that was actually a straight line angled at precisely 45 degrees.
The real world doesn't work like that, so in fact the Lorenz curve is always a concave curve, with the 10% of the population that are the lowest earners making less than 10% of the wealth, and the 10% that are the highest earners making more than 10% of the wealth -- that is, after all, what defines them as the lowest and highest earners!
The Lorenz curve can be used to graph more than just income, of course. It is also used to graph economic wealth, personal debt, ecological diversity, size of individual animals within a population, and almost any other type of variation within a population. However, in these cases it may be given a name more specific to the domain in question; for example, in measuring ecological diversity, it is more likely to be called a K-dominance curve.