Although there is no precise scientific definition, we all know it when we have it. In general, an alcohol hangover involves two or more the following symptoms: headache, nausea, diarrhea, lack of appetite, shakiness, feeling tired and an overall feeling of being unwell. Simply put, it feels like the "flu".
There are numerous changes in the balance of hormones, neurotransmitters and other biological substances in the body. This neurobiological imbalance leads not only to the symptoms described above, but also in a rapid heart rate and increased work load on the heart. The latter is responsible for increased cardiac mortality.
There is a diffuse slowing of the brain waves as seen on EEG several hours 16 hours after all alcohol has cleared the body. Neuromuscular performance is also impaired long after alcohol is undetectable in blood.
As a general rule, dark color liquors such as Scotch, Brandy, Whiskey tend to give worse hangovers than clean drinks such as Vodka of Gin. Here is a list in order of alleged "pain" they provide when one over indulges. This is based on feedback from hang over victims.