Devoir (must/to have to) as well as the impersonal verb falloir (to be necessary) indicate necessity:
Nous devons travailler pour gagner un
salaire.
(We must work to earn a salary.)
Il faut manger pour
vivre.
(One must eat to live.)
Falloir and devoir often express the same idea, although falloir is stronger than devoir:
Au cinéma, il faut se
taire.
(At the movies, one should not talk/must not talk.)
Au
cinéma, vous devez vous taire.
(At the movies, you must not talk.)
When followed by a noun, falloir means to need:
Il nous faut du
temps pour écrire ce roman.
(We need time to write this novel.)
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