I really like the one about trees creating the wind but alas it's not true.
Here is the real definition if you are still interested: Wind is the horizontal movement of air defined by it's speed and direction. So the simple explanation is that air flows from a high pressure area to a low pressure area(pressure gradient force), however in reality other forces exist. These are -
1. Pressure gradient force - described above
2. Coriolis force - an apparent deflection of moving objects when they are viewed from a rotating reference frame.
Therefore Pressure gradient force + Coriolis force = Geostrophic wind.
3. Centrifugal Force - which essentially is the effect of a clockwise rotation around a high pressure area and an anti-clockwise rotation around a low pressure area, therefore -
Geostrophic wind + Centrifugal force = Gradient wind (essentially the wind at altitude)
4. Friction force - the effect that the earths surface produces on a moving air mass - therefore -
Gradient wind + Friction force = Surface wind
After all this and you have lost all your friends you have plenty of time to get into local effects of wind such as valley winds, anabatic, foehn etc.
If only it were the trees - life would be so much simpler.