Those nerve cells lose their ability to initiate and control muscle movement, which leads to paralysis and death. People with the condition lose control of muscle movement, eventually losing their ability to eat, speak, walk and, ultimately, breathe.
ALS is also called Lou Gehrig's disease, named after the famous baseball player who retired in 1939 because of the condition. Other notable sufferers actor David Niven, NBA Hall of Famer George Yardley and jazz musician Charles Mingus.
Little is known about the causes of the disease, and there is currently no cure. The condition is slightly more common in men than women.
Hawking, diagnosed with the condition in 1963, lived with it for more than 50 years -- a remarkably long time for an ALS sufferer.
"I try to lead as normal a life as possible, and not think about my condition, or regret the things it prevents me from doing, which are not that many," he wrote on his website.
"I have been lucky that my condition has progressed more slowly than is often the case. But it shows that one need not lose hope."
Hawking's life, including his battle with ALS, was made into a 2014 biopic, "The Theory of Everything," starring Eddie Redmayne.
There are two types of ALS, sporadic, which is most common, and familial. The latter is inherited -- the children of sufferers have a 50% chance of inheriting the condition, and people with familial ALS live an average of only one to two years after symptoms appear.
But it much more rare than sporadic ALS, which accounts for over 90% of cases.
"Scientists have been studying many factors that could be linked with ALS, such as heredity and environmental exposures," the CDC says.
"Other scientists have looked at diet or injury. No cause has been found for most cases of ALS. In the future, scientists may find that many factors together cause ALS."