"The department mission statement should be the driving force behind every organization. The mission provides a focus for the organization's activities and embodies the goal that the organization strives to achieve. A mission statement is defined as a brief but meaningful statement of the reasons for the department's existence, the ways in which it accomplishes its purposes, and the values that drive the organization."
Coffee Break Training - Management Science Program Series No. MS-2013-2
United States Fire Administration: http://www.usfa.fema.gov/