SODIUM
HYDRATES:
Maintains water balance;
Activates thirst response;
Help prevent water intoxication (hyponatremia) which, in an extreme cases, can cause death.
PREVENTS CRAMPS:
Enables normal muscle contraction.
ALSO:
Influences performance of other minerals;
Enables nerve impulse transmission; Maintains normal blood pressure.

POTASSIUM
HYDRATES:
Maintains water balance.
ENERGIZES:
Stimulates metabolism of proteins & carbohydrates;
Helps muscles use glycogen, their main source of energy.
PREVENTS CRAMPS
Prevents muscle fatigue;
Enables normal muscle contraction.
ALSO:
Influences performance of other minerals;
Enables nerve impulse transmission;
Maintains normal blood pressure.

CHLORIDE
HYDRATES:
Maintains water balance;
Prevents dehydration.
ENERGIZES:
Helps the body break down protein, absorb minerals & vitamin B12.
PREVENTS CRAMPS:
Enables normal muscle contraction & relaxation.
ALSO:
Enables nerve impulse transmission.

MAGNESIUM
ENERGIZES:
Participates in the conversion of ATP (adenosine triphosphate), which are the energy packets the body uses to produce and store energy;
Stimulates the metabolism of carbohydrates & fats;
Helps the body build proteins.
PREVENTS CRAMPS:
Decreases pain from sports-related injuries & excessive physical activity;
Enables normal muscle relaxation;
Prevents muscle cramps & spasms.
ALSO:
Influences performance of other minerals;
Enables nerve impulse transmission;
Decreases vulnerability to disease;
Alleviates symptoms of numerous medical and psychiatric conditions.
Studies show that repletion of magnesium has a significant impact upon athletic performance. Taking magnesium supplements has proved to decrease blood pressure, heart rate, and oxygen intake, while improving athletic performance.
Studies have also shown consistently that most adults do not get enough magnesium in their diet.

Wikipedia paper contains basic information and includes a short discussion of sport drinks as they pertain to athletic performance.

MedicineNet paper contains information on several common electrolytes.

How Stuff Works paper contains information on the common electrolytes used by the body. Also contains several related links to How Stuff Works in the body.