A coma is a prolonged unconscious state in which the patient cannot be awakened. A patient in a coma cannot initiate voluntary activity, and can only respond minimally, at best, to stimuli.
While it is a serious condition, a coma isn't necessarily all bad. In fact, doctors sometimes even induce a coma to help with a patient's healing. A coma can be part of the natural recovery process for a brain injury patient.
The hardest part of a coma is, of course, for the loved ones watching for signs of improvement. They may become anxious for the patient to wake up and frustrated with the doctor for what seems to be a slow recovery.