It is used--
a. As a principal verb, it means to require. It means to require or want or to stand in need of.
b. As an auxiliary verb or defective verb--it means necessity or obligation. When thus used, the third person singular form is 'need', and not 'needs' in negative and interrogative sentences. It is followed by the infinitive mood and is used only in the present tense.
a. He will need money.
b. He need not have gone there.