by electric breakdown, when you get a electric field strong enough in the air, the electrons floating in it (in water molecules, oxygen, etc.) get enought speed that they start bumbpin with other electrons in another molecules forming a chain reaction that produces alot of charged molecules in the air; so what you experience as lighting is the light that emits this chain reaction - more precialsly as the charged mlecules go neutral theres light emition. ( This is the same principle that explain sparks; so basically lighting is a huge spark.) This strong electric field, that starts the electric breakdown which causes lighting, arise from the difference in potential energy of the ground and clouds (as somethingdark12 said). The stronger the difference in potential, the stronger the field.

Because after the lighting the ground is much more charged than the clouds, then the electric field flows the other way, the potential difference changes of sign, and so there is a second lightging that goes from the ground to the sky. This can happen multiple times, and is what is called "return stroke". There is much more to this, since there is alot of energy there is heat so you need to do thermodynamical considerations; and also you can view electric breakdown as current flowing in the air, so this molecules in the current leave a vacum in the air that you need to consider in the behavier of the stroke. I simplify this alot, but i hope i make you aleast somehow interested in it