This is a trick question: technically, lightning does both. Let’s take a look at the process by which lightning is known to form. This phenomenon occurs due to a charge difference between a storm cloud and the ground.
Positive lightning travels in reverse, going from the ground to the clouds.
First, the base of a cloud emits a small electrical discharge, called a step guide. It descends to the ground in steps, each about 46 meters (50 yards) long. This process is extremely fast and impossible to see with the naked eye. Each step takes less than a millionth of a second. The interval between steps is about fifty millionths of a second. This process can only be observed with the help of extremely fast exposure cameras.
The staggered leader typically moves about 75 miles per second (120 km/s) toward the ground. The typical duration of a trip is 20 milliseconds. Atoms transmit electrical charge much faster than sound vibrations.
Lightning can travel both up and down.
The stepped leader carries tons of negative charge. As it gets closer to the ground, it induces huge amounts of positive charge in the ground, especially at the tips of tall objects. As opposites attract, the staggered leader and the negative charge on the ground quickly approach and meet. The path from the storm cloud to the surface is complete and the payload can move.
Storm clouds carry an electrical charge.
Since the cloud is full of negative charge, it has plenty of current to deliver the newly created discharge path. This charge quickly goes from being distributed throughout the cloud to being concentrated at the point where the wobbly leader first fell from the cloud, to the ground or an overhead object. This discharge is called flashback and is what we think of when we hear the word “lightning”.
The return hit takes about 100 millionths of a second to hit the ground. The immense flare generated is enough to leave an afterimage on our eyes for seconds, giving us the illusion that the flare is longer than it really is. Actually, our eyes cannot figure out any of the steps involved. We only see the final product: lightning.