Almost as important as staying hydrated is knowing how to stay properly hydrated. For athletes and active individuals, sodium is the main electrolyte lost through sweat during exercise. The American College of Sports Medicine (ACSM) recommends that exercise lasting more than 1 hour should be accompanied by an electrolyte-rich beverage. Without proper sodium and water replenishment, you may experience muscle and abdominal cramping and run the risk of hyponatremia.
For more information on when to consume electrolytes, read Matt's post on When to Drink Your Calories.