What is turbulence, and how does it happen?
Air turbulence is very much the same as the currents of a river.

When a parcel or stream of air moves differently than the area around it, you get turbulence as you transition between them.  

An example would be if you are outside on a windy day but stand behind a tree to "get out of the wind".  If you step out from behind the tree, you will feel a sudden "jolt" as the wind hits you.  If it's a particularly strong wind, it could even cause you to lose your balance for a second until you compensate for it.  Well, these invisible air currents affect an aircraft in much the same way.

It isn't normally dangerous, and I think of it like driving down a bumpy road.  Of course, it's usually even safer in an airplane because even if you hit a big "pothole" you don't have to worry about it throwing you off the road into a tree.

The actual movement of the air can be caused by several things:


Wind hitting something (like a building or a mountain) and displacing it (just like rocks in a river).
Air being heated by the ground which then causes it to rise.
Weather activity like fronts, thunderstorms, or jetstreams.
An aircraft moving through the air will cause turbulence of its own (known as wake turbulence).