A mission statement is, in some ways, an action-oriented vision statement, declaring the purpose an organization serves to its audience. That often includes a general description of the organization, its function, and its objectives. Ultimately, a mission statement is intended to clarify the "what," the "who," and the "why" of a company. It's the roadmap for the company's vision statement.
As a company grows, its objectives and goals may be reached, and in turn they'll change. Therefore, mission and vision statements should be revised as needed to reflect the business's new culture as previous goals are met.
Both mission and vision statements are often combined into one comprehensive "mission statement" to define the organization's reason for existing and its outlook for internal and external audiences -- like employees, partners, board members, consumers, and shareholders.