Sweat is mainly water with small amounts of salt, potassium and other minerals that play an important role in regulating blood pressure and body's water balance. Most people sweat anywhere from a negligible amount up to two quarts a day. Heat and physical exertion increase the amount that you sweat. You obviously sweat more when you exercise because muscles burn calories, producing heat and raising your body temperature. You sweat more when it is humid because sweat doesn't evaporate very much and it doesn't cool you off.
Air movement promotes the evaporation of sweat, so a fan can help.
Sweating also makes you thirsty because it boosts the concentration of salt in your blood, stimulating the thirst mechanism. But don't wait until you are thirsty to drink water, especially when you are exercising, it may be too late. If you don't replace enough water loss, you'll sweat less and your body temperature will rise. And the more fit you are, the more easily you will acclimatize to the heat. Older people have more of a problem in the heat.