6 tourists died in Laos after apparently drinking methanol. What is it and why was it in drinks?

Two Australian teenagers and a British woman have died from suspected methanol poisoning after drinking tainted alcohol in Laos. An American man and two Danes also died, though their exact causes of death have not been released. A New Zealander also has been sickened. The U.S. State Department issued a warning to travelers to be “alert to the potential risk of methanol poisoning.” But what is methanol, and how do you avoid it?