Malic acid is also known as fruit acid or apple acid. It is found in many plants and animals, including humans. Malic acid is a biochemical product of the Kreb’s Cycle - a biochemical chain of reactions involved in the production of cellular energy (ATP). 
Malic acid has been used in the production of soft drinks and juices; as well as cosmetics due to its ability to regulate the pH and partially preserve liquids and solids.
Malic acid has recently gained recognition as a treatment for fibromyalgia. Although the exact mechanism is not fully understood, treatment efficacy is relative to its high absorption percentage in the digestive tract. Athletes are also known to use malic acid with the hope of increasing oxygen delivery and energy production in the muscle cells. This use is theoretical and has not been documented in research.
Malic acid is derived, largely, from apples and other fruits. However, the amounts available in supplement form are far greater than those achieved by eating fruits in normal quantities. Most of the supplements use malic acid from apple, as they have the greatest concentration of malic acid.
Malic acid is considered a plausible treatment for fibromyalgia when combined with magnesium. High doses of malic acid can cause a decrease in point tenderness and pain severity for individuals who suffer from this condition. 
Malic acid may also be helpful for individuals who suffer from chronic fatigue syndrome. Fibromyalgia and chronic fatigue present similar symptom patterns. However, there has not been any long-term studies to document this purported efficacy.
Malic acid at doses of 1200 - 2400 milligrams per day in divided doses, combined with 300 - 600 milligrams of magnesium, has been effective for the treatment of fibromyalgia. It is believed that individuals suffering from chronic fatigue syndrome may also gain benefit from implementing similar dosages.
There are no known side effects from the personal administration of malic acid. It does not interact with any medications, nor are there reports of overdose / toxicity.
Malic acid should be avoided in women who are pregnant or nursing due to the lack of information regarding safety in these populations. 
1. http://www.pdrhealth.com/drug_info/nmdrugprofiles/herbaldrugs/101840.shtml Malic Acid. February 2005.
2. Russel IJ, Michalek JE, Flechas JD, Abraham GE. Treatment of fibromyalgia syndrome wit SuperMalic: a randomized, double blind, placebo-controlled crossover pilot study. J Rheumatol. 1995 May; 22(5): 953-958.
3. http://www.pdrhealth.com/drug_info/nmdrugprofiles/herbaldrugs/101840.shtml Malic Acid. February 2005.