This 3D animation from The Human Atlas explains ALS/MND.

All voluntary movement in the body is controlled by the brain. Nerve cells in the brain, called upper motor neurons, initiate movement through the release of chemical signals called neurotransmitters. The signal is passed from the upper motor neurons to the lower motor neurons of the spine.

Nerve fibers extending from the spine, called axons, extend into the muscles. The point where the axons and muscle fibers connect is called the neuromuscular junction. When the signal reaches the neuromuscular junction, it causes the muscle to contract-resulting in voluntary muscle movement.

Amyotrophic Lateral Sclerosis (ALS), often called Lou Gehrig's disease, is a degenerative disease of both the upper and lower motor neurons. As more and more motor neurons become affected, they are unable to send signals for movement to the muscles. Loss of voluntary muscle movement and coordination develop. Over time, muscle weakness and spasticity develop.

Patients are unable to perform routine activities, such as walking and getting out of a chair. Eventually paralysis develops, interfering with the ability to swallow, talk and breathe.

Unfortunately, there is no cure for ALS. Death from Amyotrophic Lateral Sclerosis usually occurs within 3 to 5 years from diagnosis.