A summary of the "year without a summer" of 1816?

What was it? How this related to plate tectonics? How did it affect human Society?

The year with out a summer happened during 1816 when weather in Europe and North America took a bizarre turn that resulted in widespread crop failures and even famine. Cold temperatures returned. The lack of sunlight became so severe that farmers lost their crops and food shortages were reported in Ireland, France, England, and the United States.