I like Peter F. Drucker's definitions:
Customers — Those who must be satisfied in order for the organization to achieve results. The primary customer is the person whose life is changed through the organization's work. Supporting customers are volunteers, members, partners, funders, referral sources, employees, and others who must be satisfied.
Customer Value — That which satisfied customers' needs (physical and psychological well-being), wants (where, when, and how service is provided), and aspirations (desired long-term results).
Mission — Why you do what you do; the organization's reason for being, its purpose. Says what, in the end, you want to be remembered for.