ALS, which is also known as a motor-neuron disease—and colloquially as Lou Gehrig's disease in the U.S.—is a neurodegenerative disease. Each muscle is controlled by motor neurons that reside in the brain in the frontal lobe. These are controlled electrically and are synaptically connected to motor neurons that reside lower down in the brain—as well as motor neurons that reside in the spinal cord. The guys in the brain are called the upper motor neurons, and the guys in the spine are called the lower motor neurons. The disease causes weakness of either upper motor neurons or lower motor neurons or both.
Life expectancy turns on two things: the motor neurons running the diaphragm—the breathing muscles. So the common way people die is of respiratory failure. And the other thing is the deterioration of swallowing muscles, and that can lead to malnutrition and dehydration. If you don't have these two things, you could potentially live for a long time—even though you're getting worse.