Chronic fatigue syndrome (CFS or ME/CFS) is much more than just being tired a lot. People with ME/CFS are so run down that it interferes with their lives and can make it hard to function at all. The severity varies, but typically people with ME/CFS say they have trouble staying on top of their responsibilities at home and on the job. Others are severely disabled and even bedridden.
http://chronicfatigue.about.com/od/whatischronicfatigue/a/what_is_CFS.htm