When you're dehydrated, your body doesn't have enough fluid to work properly. An average person on an average day needs about 3 quarts of water. But if you're out in the hot sun, you'll need a lot more than that. Most healthy bodies are very good at regulating water. Elderly people, young children and some special cases - like people taking certain medications - need to be a little more careful.
Signs of dehydration in adults include
Signs of dehydration in babies and young children include a dry mouth and tongue, crying without tears, no wet diapers for 3 hours or more, a high fever and being unusually sleepy or drowsy.
If you think you're dehydrated, drink small amounts of water over a period of time. Taking too much all at once can overload your stomach and make you throw up. For people exercising in the heat and losing a lot of minerals in sweat, sports drinks can be helpful. Avoid any drinks that have caffeine.
References and abstracts from MEDLINE/PubMed (National Library of Medicine)