Alcohol in the chemistry sense is a molecule that contains a hydroxyl group. That is an oxygen bonded to a hydrogen (OH). Some examples of simple alcohols include methanol, ethanol, and isopropanol (rubbing alcohol). In more general terms, an alcoholic beverage is a drink that contains ethanol. This is an ethyl group (CH3CH2) bonded to a hydroxyl group (OH). Although ethanol is the "active ingredient," most alcoholic drinks have water as their main ingredient. Beer and wine are about 5% ethanol on average and can range up to about 10-15%. Liquors like whiskey or vodka can range from 40-90% ethanol. Other than water and ethanol, other ingredients are present depending on what type of food is fermented to make the ethanol. Beer and wine contain grain and grape sugars, respectively. Beer and wine are not distilled so they can also contain the yeasts that were used for fermentation. Also, other additives are added to the mix for flavoring (these can vary widely). Liquors are distilled, hence they are lighter and clearer in color. But even with the distillation, some flavors imparted by the fermentation process or aging process (if aged in wooden barrels, for example) will give each liquor a distinct flavor.
The chemical composition of alcohol is CH3CH2OH. There are no real 'ingredients' in it because it is a waste product of various bacterium, most commonly yeast. Due to being a waste product alcohol contains ethyl, hence ethyl alcohol. The alcohol itself is the byproduct of a process called fermentation, in which the bacterium converts various sugars into ethyl and carbon dioxide.
Alcohol that is created for human consumption is put through several different processes so that the alcohol is not tainted with any unwanted chemicals. Regardless of this, many substances can find its way into the mixture. Alcohol created for medical purposes is carefully monitored and put through a special filtration system.