I found a good explanation of bed sores on the internet: Bed sores can occur when a person is bedridden, unconscious, unable to sense pain, or immobile. Bed sores are ulcers that occur on areas of the skin that are under pressure from lying in bed, sitting in a wheelchair, and/or wearing a cast for a prolonged period of time.
Also called "decubitous ulcers" and "pressure sores". They occur when the circulation to the skin and underlying tissues is subjected to prolonged pressure and damage.