In order to understand alcohol's effects on the body, it is helpful to understand the nature of alcohol as a chemical, so let's take a look...
Here are several facts:

Alcohol is a clear liquid at room temperature.
Alcohol is less dense and evaporates at a lower temperature than water (this property allows it to be distilled -- by heating a water and alcohol mixture, the alcohol evaporates first).
Alcohol dissolves easily in water.
Alcohol is flammable (so flammable that it can be used as a fuel).

When a person drinks an alcoholic beverage, about 20 percent of the alcohol is absorbed in the stomach and about 80 percent is absorbed in the small intestine. How fast the alcohol is absorbed depends upon several things:
The concentration of alcohol in the beverage - The greater the concentration, the faster the absorption.
The type of drink - Carbonated beverages tend to speed up the absorption of alcohol.
Whether the stomach is full or empty - Food slows down alcohol absorption.

Alcohol Effects:
Men vs. Women
When you compare men and women of the same height, weight and build, men tend to have more muscle and less fat than women. Because muscle tissue has more water than fat tissue, a given dose or amount of alcohol will be diluted more in a man than in a woman. Therefore, the blood alcohol concentration resulting from that dose will be higher in a woman than in a man, and the woman will feel the effects of that dose of alcohol sooner than the man will.

After absorption, the alcohol enters the bloodstream and dissolves in the water of the blood. The blood carries the alcohol throughout the body. The alcohol from the blood then enters and dissolves in the water inside each tissue of the body (except fat tissue, as alcohol cannot dissolve in fat). Once inside the tissues, alcohol exerts its effects on the body. The observed effects depend directly on the blood alcohol concentration (BAC), which is related to the amount of alcohol consumed. The BAC can rise significantly within 20 minutes after having a drink.
Once absorbed by the bloodstream, the alcohol leaves the body in three ways:

The kidney eliminates 5 percent of alcohol in the urine.
The lungs exhale 5 percent of alcohol, which can be detected by breathalyzer devices.
The liver chemically breaks down the remaining alcohol into acetic acid.
As a rule of thumb, an average person can eliminate 0.5 oz (15 ml) of alcohol per hour. So, it would take approximately one hour to eliminate the alcohol from a 12 oz (355 ml) can of beer.

Alcohol Abuse
In the United States, approximately 8 percent of people aged 18 and older suffer from alcohol abuse and/or dependence. This abuse or dependence costs upwards of $1.7 billion in medical treatment, lost earnings, casualty damages and criminal/legal costs.
Alcohol abuse has been a rising problem over the past three decades. With the continued exposure to alcohol, how does the human body respond or adapt? The body's increased tolerance to alcohol involves the following changes:

Increase in level of liver's enzymes that are used to break down alcohol

Increase in activity of brain and nervous-system neurons
These bodily adaptations change a person's behavior.
The levels of alcohol dehydrogenase and aldehyde dehydrogenase in the liver increase in response to long-term alcohol exposure. This means that the body becomes more efficient at eliminating the high levels of alcohol in the blood. However, it also means that the person must drink more alcohol to experience the same effects as before, which leads to more drinking and contributes to addiction.

The normal chemical and electrical functions of nerve cells increase to compensate for the inhibitory effects of alcohol exposure. This increased nerve activity helps people to function normally with higher BAC; however, it also makes them irritable when they are not drinking. Furthermore, the increased nerve activity may make them crave alcohol. Most certainly, the increased nerve activity contributes to hallucinations and convulsions (e.g. delirium tremens) when alcohol is withdrawn, and makes it difficult to overcome alcohol abuse and dependence.

In addition to these adaptations, there are many adverse physical effects that result from long-term exposure to alcohol:

The increased activity in the liver causes cell death and hardening of the tissue (cirrhosis of the liver).
The brain cells in various centers die, thereby reducing the total brain mass.
Stomach and intestinal ulcers can form because the constant alcohol use irritates and degrades the linings of these organs.
Blood pressure increases as the heart compensates for the initially reduced blood pressure caused by alcohol.
Male sex-cell (sperm) production decreases because of decreased sex-hormone secretion from the hypothalamus/pituitary and, possibly, direct effects of alcohol on the testes.
Poor nutrition decreases levels of iron and vitamin B, leading to anemia.

Because alcoholics lose balance and fall more often, they suffer more often from bruises and broken bones; this is especially true as they get older.
Finally, alcohol abuse and dependence cause emotional and social problems. Because alcohol affects emotional centers in the limbic system, alcoholics can become anxious, depressed and even suicidal. The emotional and physical effects of alcohol can contribute to marital and family problems, including domestic violence, as well as work-related problems, such as excessive absences and poor performance.
While alcoholism has devastating effects on a person's health and social environment, there are medical and psychological ways to treat the problem.