There is an amazing body of academic and consultant expertise around on what organizational culture is and the place it has in the life and performance of the organization.
In my relentless pursuit of simplicity, I like to think of organizational culture as how it feels to be there. In this framework, there are more positive and negative cultures that make it more likely that people stay unengaged and behave poorly or stay engaged and behave well.
When we assess for culture, we look at the quality of conversations. Do leaders and people talk more about deficiencies, problems and blame than strengths, possibilties and passion for learning? I would argue that conversations shape culture. When we significantly transform the culture of newer and older organizations, it is always because we have transformed the character of the conversations there.
Culture is significant in that it is more powerful than structure and strategy when it comes to the performance and growth of the organization. It is that vital.