Information quoted from "How to Brew" by John Palmer

Hops are the cone-like flowers of a climbing vine that is native to the temperate regions of North America, Europe and Asia. The species has separate male and female plants and only the female vines produce the cones. The vines will climb 20 ft or more up any available support and are commonly trained onto strings or wires when grown commercially. The leaves resemble grape leaves and the cones vaguely resemble pine cones in shape but are light green, thin and papery. At the base of the petals are the yellow lupulin glands which contain the essential oils and resins that are so prized by brewers

Hops have been cultivated for use in brewing for over 1000 years. The earliest known cultivation was in Central Europe, and by the early 1500s, cultivation had spread to Western Europe and Great Britain. At the turn of the century, about one dozen varieties of hop were being used for brewing; today, there are over one hundred. The focus of breeding programs has been to maintain desirable characteristics, while improving yield and disease resistance.

Hops are a natural preservative and part of the early use of hops in beer was to preserve it. Hops were added directly to the cask after fermentation to keep it fresh while it was transported.

Beer wouldn't be beer without hops - hops provide the balance, and are the signature in many styles. The bitterness contributed by hops balances the sweetness of the malt sugars and provides a refreshing finish. The main bittering agent is the alpha acid resin which is insoluble in water until isomerized by boiling. The longer the boil, the greater the percentage of isomerization and the more bitter the beer gets. However, the oils that contribute characteristic flavors and aromas are volatile and are lost to a large degree during the long boil. There are many varieties of hops, but they are usually divided into two general categories: Bittering and Aroma. Bittering hops are high in alpha acids, at about 10 percent by weight. Aroma hops are usually lower, around 5 percent and contribute a more desirable aroma and flavor to the beer. Several hop varieties are in-between and are used for both purposes. Bittering hops, also known as kettle hops, are added at the start of the boil and boiled for about an hour. Aroma hops are added towards the end of the boil and are typically boiled for 15 minutes or less. Aroma hops are also referred to as finishing hops. By adding different varieties of hops at different times during the boil, a more complex hop profile can be established that gives the beer a balance of hop bitterness, taste and aroma.

In our recipes, I have hops listed as either bittering, flavoring, or finishing. Some recipes use all three, some two, and some only one. To give you a bit more information about them, I've added some definitions as to what makes each unique.

Bittering
The primary use of hops is for bittering. Bittering hops additions are boiled for 45-90 minutes to isomerize the alpha acids. The aromatic oils of the hops used in the bittering addition tend to boil away, leaving little hop flavor and no aroma. Because of this, high alpha varieties (which commonly have poor aroma characteristics) can be used to provide the bulk of the bitterness without hurting the taste of the beer. If you consider the cost of bittering a beer in terms of the amount of alpha acid per unit weight of hop used, it is more economical to use a half ounce of a high alpha hop rather than 1 or 2 ounces of a low alpha hop. You can save your more expensive aroma hops for flavoring and finishing.

Flavoring
By adding the hops midway through the boil, a compromise between isomerization of the alpha acids and evaporation of the aromatics is achieved yielding characteristic flavors. These flavoring hop additions are added 40-20 minutes before the end of the boil, with the most common time being 30 minutes. Any hop variety may be used. Usually the lower alpha varieties are chosen, although some high alpha varieties such as Columbus and Challenger have pleasant flavors and are commonly used. Often small amounts (1/4-1/2 oz) of several varieties will be combined at this stage to create a more complex character.

Finishing
When hops are added during the final minutes of the boil, less of the aromatic oils are lost to evaporation and more hop aroma is retained. One or more varieties of hop may be used, in amounts varying from 1/4 - 4 oz, depending on the character desired. A total of 1-2 oz. is typical. Finishing hop additions are typically 15 minutes or less before the end of the boil, or are added "at knockout" (when the heat is turned off) and allowed to steep ten minutes before the wort is cooled.