ALS is due to the death of motor neurons, the nerve cells that stimulate muscles to move. As these neurons die, muscles no longer receive commands to contract. That inability to contract is first experienced as weakness, and ultimately as paralysis.
Researchers have learned a great deal about the likely causes of ALS in recent years, although there is still much more that isnít known. We know that in some cases, inheritance of specific mutated genes causes ALS. This most often means that more than one family member is affected (called familial ALS). In most cases, however, there is no known gene, and no other affected member of the family (called sporadic ALS). There are a number of hypotheses about the causes of sporadic ALS, but as of yet none of them have been confirmed.
You can find more information about the causes of ALS, and the current state of ALS research, here.