Note: for the purposes of drink recipes on this site, "gin" refers to the London dry style. Other styles of gin will be specified (ie Old Tom).
Whoever first thought of adding juniper berries to a base spirit was a genius. This was a pioneer of the first order, with cunning creativity and good taste. This person was also the inventor of gin.
Gin originated with the Dutch, who called it genever. It was likely invented by an alchemist named Sylvius de Bouve in the early 17th century. Because distillation techniques back then were quite crude, the resulting spirit was harsh. In order the mask the harshness, herbs and botanicals were added and it was found that juniper berries were a superior additive. Juniper has been the main additive ever since, along with others such as citrus peel, angelica root, anise, coriander, cinnamon, and licorice root. The subtle differences in flavor between gins are a result of the guarded botanical recipes of the producers.
When William of Orange invaded England and assumed the throne in 1688 in the aptly named Glorious Revolution, Dutch gin came with him. The English took to gin quite quickly, and it helped that the government did not require a license to produce gin. Also, imports of gin were heavily taxed, so domestic production flourished. By the mid-18th century, gin production was at five to six times that of beer. Gin was popular with the poor because it was a cheap, strong drink.
A series of new taxes starting in 1736 led to riots in the streets. The onerous taxes were eventually rescinded and replaced with more gradual taxes and regulations that led to more government control over the production and distribution of gin. The free wheeling unlicensed days of the early 18th century were over in England.
With the invention of the more efficient column still in 1832, the London Dry style that we are familiar with today came about. It is has a higher alcohol content than Dutch gin--usually 90 proof or more.
The base spirit for gin can be made from a variety of cereal grains--wheat, rye, or barley. The grains are used to make a neutral base spirit to which flavoring is imparted from botanicals. There are two basic methods for instilling the flavors.
The Dutch tend to use barley to make the base spirit, and then will age the gin in oak barrels for up to three years. This results in a fuller body akin to whiskey. English gin is not aged, and is lighter and clear in color. Some gins are a combination such as Seagram's Extra Dry in the United States. Clear Seagram's gin sits on oak for three months and attains a straw color before it is bottled.
Gin is usually mixed with liquors, wines, or mixers. By itself, it can be a little rough, especially if it is not aged (most of it is not). The most famous mixer for gin is vermouth in a dry Martini. This is a drink that was pervasive in bars around the world in the middle part of the last century. Unfortunately, (or fortunately depending on your taste) gin has been eclipsed by vodka in the Martini.
Right after the Martini, you have the Gin and Tonic. This drink came about because the British needed something to help them gulp down their quinine, which they used to combat malaria in India and other colonies. They mixed it with gin and the rest is history. Tonic water today still contains quinine, although much less than the medicinal doses the British once used. I have found Schweppes or Canada Dry tonic water to be far superior to the cheaper brands. If you're using good gin, use good tonic.
Each brand of gin has their own recipe of botanicals and resulting flavor and aroma profile. Tanqueray has a strong juniper and cassia bark flavor and not as much citrus. Bombay Sapphire is quite pungent with hints of liquorice and almonds. Hendrick's gin is softer with rose petals and cucumber added to the botanical mix. For a very good, inexpensive gin, try Barton gin. It has a very fresh botanical flavor, especially considering the price. As you can see, there is a huge variety of flavor in different brands of gin.