Any time grains (wheat, barley, spelt, rye or oat) ferment, the result is chametz (see What is Chametz?). Many types of alcohol are made of, or contain, fermented grain and are therefore chametz.
Alcoholic beverages made of other substances are permitted. In fact there is kosher for Passover plum brandy and potato vodka. Before purchasing, ascertain that the beverage is certified as kosher for Passover.
Ashkenazim, who traditionally refrain from eating legumes on Passover (see Kitniyot), avoid alcohol produced from legumes as well.
In addition, there are some (including Chabad) who traditionally avoid all alcohol on Passover (with the exception of wine).
Rabbi Menachem Posner