What is ALS syndrome, that paralysed Stephen Hawking at 21?

Stephen Hawking was one of the most famous sufferers of amyotrophic lateral sclerosis (ALS), a fatal disease that attacks motor nerve cells in the brain and spinal cord, hampering their communication with muscles and gradually leading to paralysis. The rare condition normally claims the lives of those affected within 2-3 years, making Hawking's five-decade-long survival an extraordinary exception.