Just as for any corporation, the board of directors of a nonprofit has three primary legal duties known as the "duty of care", "duty of loyalty", and "duty of obedience". 

Duty of Care: Take care of the non-profit by ensuring prudent use of all assets, including facility, people, and, good will.

Duty of Loyalty: Ensure that the non-profit's activities and transactions are, first and foremost, advancing its mission; recognize and disclose conflicts of interest; make decisions that are in the best interest of the non-profit corporation; not in the best interest of the individual board member (or any other individual or for profit entity)

Duty of Obedience: Ensure that the nonprofit obeys applicable laws and regulations; follows its own bylaws; and that the nonprofit adheres to its stated corporate purposes/mission