Flash of lightning and lightning strike are 2 diferrent phenomenons. Indeed, flash of lightning is a discharge that occurs inside a cloud or between several clouds, where as lightning strike is a discharge between the cloud and the ground or inversely.There are about 90% of flashes and 10% of lightning strikes , so only 10% of discharges touching the ground. But we are only going to explain the mechanism of lightning strike because it’s that phenomenon that pushed humans to invent lightning rods.
It would be more logic to think that lightning strike occurs from the bottom of the cloud to the ground. But there are 25% of lightning strike that occurs from the ground to the cloud.
1)descendant lightning strike
The first phase of a lightning strike is a formation of pre-discharge called a tracer. This predischarge is not very bright. It moves on like zigzag, by leaps of dozen meters, through the neutral air, with a speed of 200 km/h. This tracer searchs the trajectory where there’s the less resistance. It’s made of electrical particles extracted from the cloud by the elctrical field cloud/ground. When the tracer comes near the ground, some sparks come out of the ground and get in contact with the tracer’s point . That tracer created a small-diameter( 2 to 3 cm) ionized canal that link the cloud to the ground. Then a discharge occurs : this is the lightning strike.
2) ascendant lightning strike
That kind of lightning strike occurs when the electrical field is intense enough to let the tracer develop from the ground to the cloud. We can note that ascendant lightning strikes are quite rare in low-relief regions, to be formed, the tracer needs a great prominence like mountains for exemple.
Franklin's Lightning Rod 