The charity board keeps order. It steers the organization in the direction of the mission statement, sets the strategic plan, sets goals and provides the resources to send the charity on its way. The founder of a charity assembles the charity board to help start and construct the nonprofit, even writing articles of incorporation, bylaws and determining the manner in which future board members will join. Organizationally, the board also hires staff from executives on down the line (so it's wise to have somebody with HR or hiring experience on the board).
One of the most important organizational tasks the board must take care of -- even when it's still just an "informal advisory committee" -- is to actually incorporate the charity as a nonprofit organization. This establishes it as a separate, legal organization, which exists separately from its founders and board, so it may continue in the event the founder or board members leave the organization. Incorporated status also protects all parties from any kind of personal liability, financial or legal.
One more thing: The number of members on your board is a legally binding figure. Check with state laws to find out minimums and maximums. (In some places, it's as low as one or even two.)