They're similar, but надо means that it is duty, нужно means that it is just an advice.

u can use нужно with verbs and nouns as long as you decline it right. But u can only use надо with verbs.

Sometimes there is an implication that надо is need due to an external circumstance, while нужно is need due to your own decision. However, this distinction does not always hold true and then these words can be used interchangeably. Also надо is a little bit more common.

Are synonymous. Previously varied dialects. Now completely interchangeable.

Надо is speaking form..