What actually happens to the human body during fasting/starvation?
Actually, the body goes into the fasting state about eight hours or so after the last meal, usually when the guy finishes absorbing nutrients from the food. 
Normally, body glucose, which is stored in your liver and muscles, is your main source of energy. When in a fasting state, your body uses up the glucose that you have stored to provide energy. If you are still fasting after this store is used up, your body will start to burn fat to create energy, therefore leading to weight loss.
After fasting for a few days, there will be a higher level of endorphin in your blood which will cause you to be more alert and, weirdly, giving you a feeling of mental well being. 
Eventually, if you are starved for too long, your body will start to break down muscle protein in order to create energy (that is why, during WWII, most POW's held by the Japanese were very thin - they were starved and in order to survive, their bodies had to 'eat' their own muscle)
Technically, starvation is when your body starts to eat your protein, i.e. Muscle.During starvation, your body will thin out, because your fat has been burned and your muscle is being 'eaten'. This obviously leads to weakness. 
When your fat is being burned, you go through a detoxification process which is caused because any toxins stored in your body's fat are dissolved and removed from the body. 
Your kidneys are very efficient in being able to maintain the body's water and salts, like potassium and sodium. But these can be lost through perspiration. 
This eventually leads to dehydration which does many things to your body, like: not allowing for your body to regulate body temperature; your heart works harder (most of your water is in your blood (about %$60$) and when you lose the water, your heart has to 'catch' up and pump more to keep the blood flowing and cooling you down - now, your heart is working harder which could give you a greater risk of heat exhaustion); you can feel cranky, drowsy, confused, forgetful.
These are a lot of things that happen when your body is deprived of what it needs in terms of food and liquid. 

This is what happens, and this is very general, when your body is in its starvation mode. I am sure that there are many more things, but I could only give a brief answer.