Over the years, I've probably read 15-20 explanations of the difference between a mission and vision statement, and this is the best one I've read. Clear and well explained.
This is an excellent and important question. Think of the words 'visionary' and 'missionary'. A visionary is someone who imagines possible futures. A missionary is someone who does work under the guidance of a larger objective. Similarly, the vision statement describes how the world will be different because of the existence of the group, while the mission statement provides the 'vehicle' for the vision; it describes the means that will be used to achieve the desired future. Because of their nature, the vision statement generally lasts for the life of the organization, while a mission statement should be revisited every two to three years to make sure that the means being used to attain the vision are still relevant.
Me: Vision statement = what you want the world to look like; Mission statement = how you intend to get there.