In 1990, the American College of Rhematology published the first official diagnostic criteria for the disorder.  Fibromyalgia literally means: fibro (fibrous tissue) myo (muscle) algia (pain). The ACR committee determined that a diagnosis of fibromyalgia would depend on the presence of all of the following:

Widespread pain for at least 3 months defined as:

--pain on the right and left sides of body
--pain above and below the waistline including shoulder and buttock pain
--pain in the distribution of the axial skeleton - cervical, thoracic and lumbar
    spine, and anterior chest wall
--tenderness to palpation with 4kg force (a blanched thumbnail upon
    downward pressure) at 11 of 18 discrete areas known as tender points

Fibromyalgia is a central nervous system disorder. The central nervous system is comprised of the brain and spinal cord. Fibromyalgia occurs when the brain and spinal cord don’t communicate properly with the rest of the body. There is more to fibromyalgia than the experience of chronic widespread aches and pain.   Many researchers believe Chronic Fatigue Syndrome and fibromyalgia symptoms overlap.  CFS case definitions are:

6 months of new onset fatigue that reduces function, unexplained by other
    medical illness or major psychiatric co-morbidity
post exertional relapse/malaise
attention/concentration and cognitive complaints
unrefreshing sleep
arthralgia (joint pain)
myalgia (muscle pain)
painful lymph nodes
new headaches
sore throat

Fibromyalgia is a chronic illness typically characterized by its symptoms: widespread pain and fatigue. Some people who have fibromyalgia may complain only of pain. Most people, however, experience a variety of symptoms and syndromes that fall outside the definition of fibromyalgia, yet frequently accompany it. These are additional symptoms of fibromyalgia:

anxiety and/or panic attacks
chronic fatigue and low energy
depression
intolerance to hot or cold temperatures
irritable bowel syndrome
neck and back pain
poor circulation (cold hands and feet)
cardiovascular problems (dizziness, palpitations)
tension headaches and/or migraines
post exertional pain
tenderness
weakness
sleep disturbance
attention, memory and concentration difficulties (called fibro fog)
allergy-like sensitivity to drugs or medications
sensitivity to loud noises
muscle cramps
stiffness (especially upon awaking)
 

Regional or organ-specific symptoms and syndromes and many other conditions are related to fibromyalgia and fall under the umbrella of chronic multi-symptom illnesses. Associated fibromyalgia conditions are:

tension/migraine headache
IBS (irritable bowel syndrome)
interstitial cystitis
chronic fatigue syndrome
multiple chemical sensitivity
TMJ/TJD
Restless Leg Syndrome
pelvic pain in women (painful menstrution and/or PMS)
sleep disturbances
muscle twitching
subjective soft tissue swelling or paresthesia in hands, arms, feet, or legs
somatoform disorders
orthostatic instability
Gulf War illness
POTS (Postural Orthostatic Tachycardia Syndrome)
 

(References: 
University of Michigan Center for Chronic Pain & Fatigue Research Center http://www.med.umich.edu/1libr/aha/umfibromyalgia.htm
O.F.F.E.R. (Organization for Fibromyalgia & Fatigue Education and Research) http://www.offerutah.org