Exercise depletes the body's stores of fluids and minerals, which must be replaced. It has been determined by most experts that if you engage in light to moderate exercise, drinking water will do this fine. However if exercising strenuously, you'll also need to replenish some of the salts that your body loses through sweat. These salts, also known as electrolytes, are found in most sports drinks, and also in natural juices like orange juice.
The proper concentration of electrolytes in your blood is essential to your health, for example your cardiovascular and nervous system require electrolytes to function well. If you were lost in the desert and drank nothing but water then you would still die, as you won't have replaced the salt (sodium) lost from your body through sweating.
We're not keen on energy drinks as they are mostly filled with artificial sweeteners, however a quick and healthier alternative is to mix equal amounts of fresh orange juice (or any citrus fruit) with water (preferably distilled or boiled and cooled), approx 500ml of each and a pinch of salt, (you can add 1/2 tsp of honey to taste if required) mix/stir well.