Chronic Fatigue Immune Dysfunction Syndrome (CFIDS) is a serious and complex disease that is still not well understood. The symptoms can include extreme fatigue, muscle and joint pain, impaired immune and endocrine systems, confusion, depression, weakness, neurally mediated hypotension, neurological problems, swollen and/or painful lymph nodes, unusual sensitivity to drugs, foods, and/or chemicals, and other things. A complete explanation of it can be found at the CFIDS Association of America.
So far there is no medical cure, although there are medications that help some people with some symptoms.
The doctor I have now feels that CFIDS is actually part of a larger syndrome that also includes fibromyalgia. From what I understand, if you go to a doctor with a certain cluster of symptoms but a primary complaint of fatigue you are more likely to be dx'ed with CFIDS. If you present with exactly the same cluster of symptoms, but complain primarily of pain, you are more likely to be dx'ed with fibro.
Here are a couple sites that have more info: