Grain alcohol (also called ethyl alcohol or ethanol) is pure alcohol, and is made from grains like corn and rye that have been fermented and distilled. And no, this is not something you can pour into a mixed drink; grain alcohol is toxic, and is not even used at 100% concentration. It is quite powerful, and is primarily used as a solvent or in other industrial products.
Grain alcohol is a component in beer, however, and in neutral grain spirits like Everclear. In these cases only a small amount is used, or it is merely a base. Pure grain alcohol is not usually sold to consumers, and some countries ban the commercial sale altogether. In fact, alcohol manufacturers usually process the alcohol in such a way that the grain is not fully fermented, or they add other liquids to dilute the grain alcohol content (this is the case in vodka).
Grain alcohol has many adverse affects on the body. It can impair brain and motor functions, though some people get a kick out of the “fuzziness” or lightheadedness that they get after ingesting it. However, taking grain alcohol regularly over a long period of time has permanent and sometimes lethal effects. It can damage the brain and the liver.
Grain alcohol can also be used in industrial products, such as fuel, or as a solvent. It is also used as an antiseptic, though it should not be confused with isopropyl alcohol. These forms of grain alcohol should not be used alternatively with beers or vodkas; they contain additive chemicals that are dangerous to consume.