The short answer is widespread muscle pain. There are specific "tender points" where just slight pressure causes pain. There are no diagnostic test to confirm this diagnosis. Along with this pain, many suffer with additional symptoms such as fatigue, pain in muscles after use can make some people nearly bedridden. Some physician's think its an autoimmune disorder. Over sensitive nerves under the skin. Causes are unknown. Some have experienced these symptoms after a traumatic injury, or perhaps a stressful event. There is no cure only symptom management.
Go to google and type in "fibromyalgia".