Devoir and Falloir can be confusing because they both express obligation and necessity, but in different ways. In addition, each verb has a different meaning when followed by a noun. Devoir - When followed by an infinitive, devoir expresses obligation, probability, or supposition. Je dois partir I have to, I must, I'm supposed to leave Falloir - stronger and somewhat more formal than devoir; it expresses necessity. Falloir can be used with an infinitive or the subjunctive. Il faut travailler It is necessary to work, One needs to work.

whatever Josh-dal said is correct but i add another thing too. "devoir + infinitif" has another meaning. It refers to future, for example u say: Mon ami doit arriver à al gare de Lyon ce soir à 9 heures.