The term legume has two closely related meanings in botany, legume can refer to either the plant itself, or to the edible fruit (or useful part).
Legumes are noteworthy for their ability to fix atmospheric nitrogen, an accomplishment attributable to a symbiotic relationship with certain bacteria known as rhizobia found in root nodules of these plants.
The ability to form this symbiosis reduces fertilizer costs for farmers and gardeners who grow legumes, and means that legumes can be used in a crop rotation to replenish soil that has been depleted of nitrogen.
Legume seed and foliage has a comparatively higher protein content than non-legume material, probably due to the additional nitrogen that legumes receive through nitrogen-fixation symbiosis.
This high protein content makes them desirable crops in agriculture.