It is a Clinical Condition which occur when excessive amounts of water or low sodium or hypotonic solution are takeen or given by any route.

It can occur in the following situations
Over load of intravenous 5% glucose solution.
Colorectal washout with plain water.
In appropriate anti diuretic hormon secration in Lober pneumonia, Emphyema, Oat cell carcinoma of Bronchus, Head injury.
Taking of excessive amount of water.

I am not a medical professional but I have a background in biology. I am not going to give a full answer, just a nutshell in plain English. You are composed mostly of water, but there is a delicate balance that your body maintains between salts and water. If the proportion of water to salt is too high or too low it can kill you. Usually this is not a problem because your body is good at maintaining the balance. However, sometimes athletes, who have lost a lot of salt through sweat, drink too much plain water too quickly and die. This is why athletes drink better balanced sports drinks to recover during or after strenuous exercise. It used to be a very rare tragedy but has become more common with the intensity of modern sports and the participation of many novices.

It is not only athletes that have to be reasonably careful. In a notorious recent incident a lady died trying to win some minor prize at a radio station. They had persuaded contestants to drink large amounts of water (without going to the bathroom) in a sick attempt at humor. I doubt that anyone, doctor or otherwise will contradict me on this: Don't ever do any obviously stupid thing on a dare. Too much of anything will kill, stick to natural amounts.

Water intoxication is the name of this condition when you have imbibed too much water and the salt concentration in your blood is too low.

Hyponatremia, also called water intoxication, is generally the result of drinking excessive amounts of plain water which causes a low concentration of sodium in the blood. Once a rare occurrence at sporting events, it is becoming more prevalent as participation increases and more novice exercisers are entering endurance events.
Prolonged and excessive sweating increases the risk that an athlete will alter the delicate balance of blood-sodium concentration. Because sodium is lost in sweat it is important for those exercising at high intensities for long periods of time to replace any loses.

Research has found that long duration endurance events, such as the Ironman distance triathlons, often have finishers with low blood sodium concentrations. Those at most risk are those who are on the course the longest, because they tend to drink the most water during the event. Runners who drink extra fluids in the days before the race or those who stop at water stop during the race are also at increased risk of hyponatremia. In fact, a study published in the New England Journal of Medicine (April, 2005) found that 13 percent of Boston Marathon runners developed hyponatremia from drinking too much water.

Causes of Hyponatremia
During high intensity exercise, sodium is lost along with sweat. An athlete who only replaces the lost fluid with water will have a decreased blood-sodium concentration. As an example, consider a full glass of salt-water. If you dump out half of the contents of the glass (as is lost in sweat), and replace that with water only, the sodium concentration of in the glass is far less and the water is more dilute. This can occur in the bloodstream of an athlete who only hydrates with water during excessive sweating. The result is hyponatremia.

Adequate sodium balance is necessary for transmitting nerve impulses and proper muscle function, and even a slight depletion of this concentration can cause problems. Studies have shown that high intensity athletes can lose up to 2 grams of salt per liter of sweat. Replacing this during the event is critical to performance and safety.

Symptoms of Hyponatremia
The early warning signs are often subtle and may be similar to dehydration and include nausea, muscle cramps, disorientation, slurred speech, and confusion. At this point, many athletes drink more water because they think they are dehydrated. Unfortunately, water alone will increase the problem of hyponatremia. At the most extreme an athlete may experience seizures, coma, or death.

Treatment of Hyponatremia
At the first sign of symptoms an athlete should drink a sodium containing sports drink or eat salty foods. Ideally, an athlete should plan ahead and estimate his or her fluid loss and need for sodium replacement during the event, and stay on a hydration schedule during the race. If the symptoms are extreme, a medical professional should be seen.

Preventing Hyponatremia
The best way for an athlete to avoid such problems is to plan ahead by training in the same conditions you will encounter during race day. Hydration recommendations include:

Use a sodium containing sports drinks during long distance, high intensity events (more than 60-90 minutes long).
Increase salt intake per day several days prior to competition (except for those with hypertension).
Try not to drink more then you sweat.
During a marathon a good rule of thumb is to drink about 1 cup of fluid every 20 minutes.
In the days before the race, add salt to your foods (provided that you don't have high blood pressure and your doctor has not restricted your salt intake).
Avoid use of nonsteroidal anti-inflammatory (NSAIDS) medicines that contain sodium. Research suggests that these drugs may predispose runners to hyponatremia.
Keep in mind that all athletes respond differently to exercise; fluid and sodium needs will vary accordingly. Foods that provide additional sodium include chicken noodle soup, a dill pickle, cheese, pretzels, and tomato juice.

As always, it is important to consult your physician for special considerations if you have a history of any health problems or are taking any medication for a health condition.

Water intoxication is also known as Hyponatremia. It is an abnormally low concentration of sodium in your blood.

Hyponatremia is typically the result of a medical condition that impairs the excretion of water from your body, or the excessive consumption of water.

One of the causes of Hyponatremia is consuming excessive water during exercise. Because you lose sodium through sweat, drinking too much water during endurance activities, such as marathons and triathlons, can dilute the sodium content of your blood.

Further, the kidneys of a healthy adult can process fifteen liters of water a day. You are unlikely to suffer from water intoxication, even if you drink a lot of water, as long as you drink over time as opposed to intaking an enormous volume at one time. As a general guideline, most adults need about three quarts of fluid each day. Much of that water comes from food, so 8-12 eight ounce glasses a day is a common recommended intake.