A mission statement is a brief description of a company's fundamental purpose. It answers the question, "Why do we exist?"
The mission statement articulates the company's purpose both for those in the organization and for the public.
For instance, the mission statement of Canadian Tire reads (in part): “Canadian Tire is a growing network of interrelated businesses... Canadian Tire continuously strives to meet the needs of its customers for total value by offering a unique package of location, price, service and assortment.”
The mission statement of Rivercorp, business development consultants in Campbell River, B.C., is: “To provide one stop progressive economic development services through partnerships on behalf of shareholders and the community.”
As you see from these two examples, mission statements are as varied as the companies they describe. However, all mission statements will "broadly describe an organization's present capabilities, customer focus, activities, and business makeup" (Glossary, Strategic Management: Concepts and Cases by Fred David).
The difference between a mission statement and a vision statement is that a mission statement focuses on a company’s present state while a vision statement focuses on a company’s future.
Every business should have a mission statement, both as a way of ensuring that everyone in the organization is "on the same page" and to serve as a baseline for effective business planning. See How to Write a Mission Statement to learn how to write one of your own.