The study of damage to a human caused by lightning is called 'keraunopathy'.
When a human is struck by lightning, there are two forces that cause damage.
The first is the heat of the blast. Lightning is extremely high temperature. It will cause the victim to breathe in superheated air and burn the skin and clothes. Hot lightning is extremely rare. It is very high voltage and most dangerously, can remain charged for up to a second. Most lightning lasts less than a tenth of this time. Hot lightning can melt pretty much anything, including steel and stronger metals. Because it lasts so long, huge amounts of current are pumped into the area affected. A human being would be utterly destroyed by such a blast. Lightning may also turn sweat and water into steam, blowing clothes of the body and quite literally poaching someone alive. The heat also focuses on metal on the body, such as keys, belt buckles, spectacles and the like.
The second form of damage lightning does is purely electrical. Humans, of course, run on electricty and dumping so much electricity into it is dangerous. The most common form of death from lightning is actually a cardiac arrest. The heart simply stops beating. Despite popular fictions, humans don't hold the charge so they can receive immediate care. Myocardial infarcs are also common effects, as is severe arrythmia.
The central nervous system, which is basically a stream of electricity, is also endangered by lightning. The brain and the spine can suffer massive damage during a strike. This can lead to brain damage, a host of nerve damages and electrochemical systems damage. Long nerves in the body can be completely 'blown out'. This can lead to things like the brain forgetting how to breathe as respitory centres are destroyed. Apparently up to 20% of all lightning deaths are via this rather ghastly mechanism.
Even if a human survives lightning, which to be fair is between 60-80% of the time, research varies, there are many conditions that can be caused, including Parkinson's Disease, liver damage and cornea and ear drum damage.
Most fascinatingly, a fairly common wound is called the Lichtenburg Figure, a very early example of fractal, that are by themselves fascinating.