How Lightning Works ?

Although there is still some debate in the scientific community about how the electrification of clouds actually occurs, it is agreed that the separation of positive and negative charges must occur within a cloud for lightning to take place. It is also generally agreed that ice must be present within a developing storm for it to eventually form lightning.
The turbulent wind environment of a thunderstorm with its updrafts and downdrafts is an ideal environment to separate electric charges: negative charges generally gather near the base of the cloud, while positive charges build in the upper reaches of the cloud. This allows electric fields to form and grow between the cloud and the ground and within the cloud itself - all necessary conditions for lightning to occur.

Since similar-charged objects repel each-other and opposite-charged objects attract each other, negative charges then begin to spread out near the base of the cloud. At the same time, positive charges start to build underneath the storm. This region of positive charges travels underneath the cloud, almost like a shadow. The positive charges tend to concentrate on tall objects, like trees, poles and buildings.

A cloud-to-ground lightning strike starts as a channel of negative charges makes its path towards the ground. This occurrence is known as a stepped leader. The stepped leader continues towards the ground in a series of steps that are each about 50 to 100 metres in length. This stepped leader can branch out in many directions.
In response to the discharge of negative charges coming from the cloud base, currents of positive charges start moving upward from the ground, usually along elevated objects; these are called streamers or upward leaders.
When the stepped leader and the upward leader meet, usually between 30 to 100 metres above the ground, the negative charges begin to flow downward. Almost instantaneously, a much larger and luminous electric current shoots up to the cloud, following the path taken by the stepped leader. This is known as the return stroke, and it is also what we see in the sky that is known as lightning. This whole process occurs so quickly (in less than one second!) that the lightning appears to travel from the cloud to the ground, when in fact, the opposite is true.
A lightning strike, or what Environment Canada refers to as a lightning flash, is usually made up of more than one stroke of lightning. After the initial strongest stroke, subsequent weaker strokes can follow which usually (but not always) follow the same path of the initial stroke.  When the lightning follows the same path, you’ll notice that it appears to flicker. If it takes a bit of a different path, it can appear to dance. On average, there are usually three to four subsequent strokes but the number can be as high as over twenty.
Examine above the photo of lightning hitting a tree. The stepped leader has obviously connected with the upward leader that came from the tree. But if you look closely, you can also see an upward leader coming from the tree that did not connect (indicated by the red arrow). There is also a very faint upward leader visible, coming up from a telephone pole just to the left of the tree (marked by the yellow arrow). These occur too fast and are often too faint for our eyes to see, other than when they are caught on film like this photographer managed. This photo also clearly demonstrates why it is not wise to seek shelter under a tree during a thunderstorm!
Cloud-to-ground lightning is the most dangerous and damaging form of lightning. Despite this, lightning within a cloud or that goes from cloud-to-cloud is the most common type of lightning. This type of lightning looks like a bright flicker in a cloud. For every cloud-to-ground strike, there can be three to five cloud-to-cloud strikes.
 source : https://www.ec.gc.ca/foudre-lightning/default.asp?lang=En&n=9353715C-1