Thunderclouds are charged with a large amount of negative charges. The charges arise due to the friction between the water molecules and air molecules as the wind shifts them around.
The Earth is at a higher potential than the clouds, hence the negative charges in the cloud will want to go to the ground. These negative charges will flow through the ionized air (conductive) to the ground with the shortest path possible. During this process, the air is heated up rapidly which produces the light (lightning) and sound (thunder).