The family leguminosae is now split into three families: leguminosae, fabaceae (aka papillionaceae)and mimosaceae. Books will refer to different names depending on when it was printed. Either way, both house certain bacteria within their roots which help fix nitrogen from the atmosphere (which plants cannot use) into nitrates (which plants can use) into the soil. This process is nitrogen fixation, and helps to replenish the amount of nitrogen in the soil. Hence, cover crops are often plants within this family (or these families).
Characteristics of Fabaceaa:
Pinnate leaves or bipinnate (rarely other leaf forms)
Regular or irregular
flowers (Clitoria sp.) --> sweet pea
flower shaped
seed pods usually one chambered