A recession is a widespread decrease in economic activity. Such decrease usually causes many employees to be unemployed. A very serious recession is referred to as a depression. Causes for recession have been tied to excess inventories, decrease in consumption (attributable to fears about the future, for instance), lack of innovations and new capital formation, and random shocks.
The worst recession on record is that of the 1930's. About one person out of 4 (or close to 25%) was unemployed. The hardship was more than just a lack of income, and, for some, consisted in broken lives and families.