Alcohols are organic molecules assembled from carbon (C), oxygen (O), and hydrogen (H) atoms. When 2 carbons are present, the alcohol is called ethanol (also known as ethyl alcohol). Ethanol is the form of alcohol contained in beverages including beer, wine, and liquor.
Alcohol is that fourth, but nonessential, macronutrient. It provides energy in the form of calories, but is not necessary for sustaining life. There are 7 calories per gram of alcohol, so alcohol sits between carbs and protein (both 4 cal/gram) and fats (9 cal/gram) as far as calorie density goes.