Nonprofits and nonprofit boards in particular have a culture -- a way they operate that is more often than not one of those "you know it when you see it" -- essentially informal far more than formal (written and decided upon by the body).
Nonprofit cultures can be very dominant defining how members should address each other and can easily close off or exclude new board members or those who some think aren't "proper".
Cultures are driven by values. I think this piece from the Conference Board Review, "What Do You Value" by Kevin and Jackie Freiberg with Dain Dunston, is very helpful in both describing culture and its importance.
Corporate culture is an expression of the company's personality and genetic makeup--its DNA. Culture reflects the rites, rituals, traditions and values that both describe and instruct how you handle big ideas, treat big thinkers, take extraordinary risks, and respond to dramatic change. Culture isn't something that is peripheral or collateral to the business-it's your very way of doing business. Thus, culture touches everything, influences everything, and affects everything.
Bottom line: Culture matters and not as an obstacle to but as a vehicle for achieving mission.