There are many different websites and answers to the basic question; What is a strategic plan?  However, I prefer the following by alliance online the best:

Strategic planning is a management tool, period. As with any management tool, it is used for one purpose only: to help an organization do a better job - to focus its energy, to ensure that members of the organization are working toward the same goals, to assess and adjust the organization's direction in response to a changing environment. In short, strategic planning is a disciplined effort to produce fundamental decisions and actions that shape and guide what an organization is, what it does, and why it does it, with a focus on the future. (Adapted from Bryson's Strategic Planning in Public and Nonprofit Organizations) 
Source: http://www.allianceonline.org/FAQ/strategic_planning