can any one plz say me the significance of " back e.m.f. " in case of d.c. generators....plzzz do reply to me at " #-Link-Snipped-# "

can any one plz write to me the significance of "back emf" in came of d.c. motors....plz write to me at " #-Link-Snipped-# " .... plzz do reply soonnnnnn