ALS, also known as amyotrophic lateral sclerosis or Lou Gehrig's disease, is a progressive neurodegenerative disease that affects nerve cells in the brain and spinal cord, causing muscle weakness and eventually paralysis. It's characterized by the degeneration and death of motor neurons, which are responsible for sending signals to muscles, leading to a loss of voluntary movement and coordination. ALS is a fatal disease, and while the average life expectancy is 2-5 years, some individuals live much longer.