Ethanol is the alcohol that is found in beverages and is produced as a by-product of fermentation of yeast and sugar. It is colorless and tastless - many high proof beverages are often called 'smooth' or said to go down like water (another colorless and tasteless beverage). This sugar is often part of some grain, thus the term 'grain alcohol'. To an animal (including the poor yeast that were killed as part of producing it), alcohol is a poison and the body does everything it can to make it less of one.
The first step in metabolizing ethanol is found in the stomach and is the enzyme 'alcohol dehydrogenase'. An interesting point with this enzyme is that it differs between the sexes - the same enzyme found in men is on the order of 70%-80% more effective than that in women. This difference also becomes more pronounced in young women and men over the age of 50. Frequent drinking also reduces the body's supply of this enzyme.
Alcohol dehydrogenase (often called just 'ADH') coverts ethanol to acetaldehyde - which is even more toxic than ethanol. This is then converted into acetate and various other compounds. While ADH is named after alcohol, this enzyme also processes other compounds too, such as: retinol, steroids, and fatty acids. There are actually 9 forms of this enzyme within the body - each composed of two parts (each part may be one of three chains - alpha, beta, or gamma -- 32 = 9). Furthermore, this enzyme works on a wide range of alcohols too, which can be a problem. When processing methanol, ADH converts it to formaldehyde which then damages various proteins - especially those in the retina leading to blindness. It should be noted that the treatment for several types of poisoning related to the post metabolic product (methanol, anti-freeze, and others) created by ADH is to drink lots of ethanol which is handled first by ADH, thus using up all of the ADH and allowing the not quite so poisonous compound to be flushed from the system.
As most people know, eating food along with drinking reduces the impact of the ethanol. This is because the sphincters in the stomach is closed to digest the food and gives the enzyme more time to work on the ethanol before it is passed to the intestines where the remaining ethanol will be rapidly absorbed by the body.
Once the ethanol gets into the blood stream, there are two interesting places for it to go. The first is the brain. Within the brain, ethanol acts as a stimulant and a depressant, becoming more of a depressant as the level of ethanol increases. The 'buzz' is from an increased activity in the pleasure center of the brain (similar to cocaine). Furthermore, ethanol reduces activity in
the anxiety centers of the brain which leads to calming, and reduced anxiety (much akin to Valium). Yet another effect upon the brain is in the serotonin (a neurotransmitter) system similar to Prozac which reduces depression and increases self-confidence. It should be noted that these effects in the brain are addictive and lead to alcoholism. Higher levels of ethanol interferes with the brain in many areas - from reduced control over muscles (clumsy, coordination, and slurring of speech) to the formation of memories. Driving in this state of mind is drunk driving, and between the reduced control of muscles and other interferance, can be very dangerous to the driver, passengers, other drivers and people on the sidewalk. Even higher levels of ethanol cause parts of the brain to shut down and is known as alcohol poisoning.
The other interesting place in the body is that of the liver. The liver's purpose in life is to break down poisons. A healthy liver in a young adult male will take on the order of one hour to process a drink - it will take longer in a young woman. Drinking faster than the liver can process the alcohol leads to getting drunk. Realize that the liver can only do so much work - other chemicals that are metabolized in the liver such as anti-histamines and other drugs will result in something not getting done. This can either reduce the effectiveness of the drug (such as antibiotics), damage the liver (as with anti-histamines and Tylenol), or even more serious effects (see Don't mix acetaminophen with alcohol).
http://www.alcohol.org.nz/effects/women/happens.html
http://www.rcsb.org/pdb/molecules/pdb13_1.html