Brief 'splanation, Lucy.
The AWG was based on circular mil area. The starting point was #10 wire, which, when it was initiated, had a cma of 10,000 circular mils. It has since been redefined as a cma of 10,380, no, I can't really explain why. ( I do understand it has to do with more accurate measuring equipment, but....).
A more detailed explanation can be found in 2-5 of the American Electricians Handbook, it does go into an explanation of weights and resistances and how they were chosen. The Brown and Sharpe gauge is the same, and Table 2-85 gives a listing of other gauges and their equivalents.