The reason you feel ill after drinking a lot of alcohol is the build up of toxins in your system. Alcohol contains many toxins such as methanol, which irritate your stomach. Also, when you drink, enzymes metabolize the alcohol
(ethanol = CH3-CH2-OH) in your system into aldehydes. Your system will slowly remove these from your system, but until then you feel sick. Alcohol is a diuretic, but dehydration only adds to the ill feeling you have. The proof is that you will be more dehydrated after sweating a lot from physical activity, but you just feel uncomfortable, not sick. Smoking eases the pain the toxins cause and is thus the only really good cure for a hangover.