If we cannot even get clarity and differentiate between Vision and Mission, then we will find difficulty engaging people in the strategic direction of the business. In worse cases, I have seen many people/companies get the Vision and Mission all twisted in reverse.
A Mission tells the reason why the company/entity exists - purpose or reason for being.
A Vision explains what the organization wants to become in the future.
The best way to explain this is always to give examples. Let's take dariyahussine's example - Centers for Disease Control. I wholly agree with the Mission, but not with the Vision.
We acknowledge a vision as a long-term goal, aspiration, or destiny that we want to achieve in the future. But how come we just treat it as some "infinite" wishful thought devoid of substance in driving towards its achievement? The biggest flaw in vision statements is when you do not give it a timeframe - deadline!
"Healthy people in a healthy world" sounds superfluous, impractical, and overgeneralized. How would one reword/rephrase it to better reflect the future state it wants to see?
I would say, "to become the first disease control agency in the world to achieve 100% control of Malaria by 2015" (Let's assume, currently Malaria happens to be the biggest killer disease).
Now, tell me what goes through your mind as an Employee! We often forget to put our ideas and decisions through this test process. The Vision is for everyone to lock and align their focus, commitment, tasks to that overarching "ultimate" goal written under Vision. This is what gives direction, meaning, motivation, morale, and substance to the job, tasks, activities that people come to work for. The entire workforce can then be geared to rally behind this single big agenda, in a selfless state of engagement.
If we can't get the Vision "right," how can we expect our people to be on the same page in the execution phase? Year in and year out, we may seem to be making significant progress in accordance with our stated Mission, but how do you measure the "effectiveness" of your progress in that journey if you have not set some checkpoints?
A Vision is a goal - hence should follow the tenets of SMART principles and it must have a timeline. Once you have achieved it, then set another compelling Vision to stretch the capability and potential of people. Vision is all about people - giving people hope, direction, and purpose to stay challenged. We should avoid making it a "meaningless" statement to satisfy having something on the wall.
Now, how complicated can that be.
Hope the above helps.
Rgds
Yuva