Why does lightning form in a zig-zagged line?

Ever wondered why lightning never forms in a straight line? The reason is simple.

This is similar to the flow of river.  The water of a waterfall goes in a straight line because there is no obstruction to it. But the water of a river on a slope follows a zig-zag line called meandering.  In lightning that phenomenon is called tortuosity.

Lightning is just an electricity. A huge chunk of charged electrons trying to find the path of least resistance to reach the ground. The electric arc forms where the air can be easily ionized and generally in the direction of shortest path between the electric field and the ground.

To know more about how does the lightning know where to go  click on this thread on Stackexchange.
http://physics.stackexchange.com/questions/55340/how-does-lightning-know-where-to-go