OG stands for “Original Gravity” which is the specific gravity (sugar) of the unfermented wort. The primary contribution to specific gravity is sugar, some of which is fermented into alcohol, and some of which remains in the finished beer to give sweetness and body. A specific gravity of 1.040 corresponds to a 10% (by weight) sugar solution, and will produce, on average, about 4% alcohol (measured by volume).
FG stands for “Final Gravity” which is the specific gravity of the finished fermented beer. It will always be less than the original gravity because during fermentation heavy sugars are converted to lighter carbon dioxide and alcohol. The gravity is reduced both by the reduced sugar content, and because alcohol is lighter than water. Thus the sugar content of the finished beer cannot be calculated directly from the final gravity, without taking the alcohol content into account.
HG stands for “High Gravity” beer, often called “big beer,” which is bold, flavorful and strong. Everything about these beers are substantial, from their intense flavor and other exaggerated characteristics to their generally higher alcohol content which the percentage will vary from state to state for what is considered high gravity by law.
“High-gravity” refers to brewing a beer with high original gravity (OG) typically, above 1.075 OG is considered high. OG is a measure of the fermentable and un-fermentable substances in the wort before fermentation. This is measured after the initial boil, before the yeast is pitched, and will be used later in conjunction with the final gravity reading to calculate the alcohol percentage.
High OG means there is plenty of “food” (sugar and other nutrients) in the wort for the yeast to eat. When yeast eat, they produce, among other components, alcohol. Given the right conditions, atmosphere, and care, certain yeast strains in high-gravity wort are capable of producing very large amounts of alcohol, which is why the “high-gravity” descriptor is synonymous with a high alcohol percentage.
A few common beer styles with High Gravities are Barley Wine, Imperial Porter, Imperial Stout, Scotch Ale, Imperial IPA, Wheat Wine Ale, Belgian-Style Golden Strong Ale, and most barrel aged beers.