According to National Geographic:
The map shows population density; the brightest points are the highest densities. Each country is colored according to its average annual gross national income per capita, using categories established by the World Bank (see key below). Some nations— like economic powerhouses China and India—have an especially wide range of incomes. But as the two most populous countries, both are lower middle class when income is averaged per capita.

The feature article, Age of Man, is worth perusing, as well. If you’re a visual learner, you’ll also enjoy The Face of Seven Billion, where you can explore how the global population breaks down in terms of language, nationality, literacy, religion, and so on.

Related posts:

1. Data Visualization: Capital Overload?
2. Demographics Driving Wealth
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4. Data Visualization: People Moving
5. How Much Your State’s Population Grew (Or Shrank) In The Past Decade