Beer, wine, and hard liquor (distilled spirits) all contain alcohol. The following common alcoholic drinks contain equal amounts of alcohol and are often referred to as a drink or a standard drink:
Proof is the amount of alcohol in hard liquor or distilled spirits. The percentage of pure alcohol in the hard liquor is usually one-half the proof. For example, a 100-proof liquor is about 50% pure alcohol. Thus, the higher the proof, the more pure alcohol the hard liquor contains.
WebMD Medical Reference from Healthwise