Gin is a category of spirits that is based in neutral spirits. The most typical base is grain spirits which is essentially the same as vodka. What gives gin its distinctive characteristic is that it is flavored with Juniper. The name gin itself is derived from the Dutch name for juniper, "genever." Juniper berries have been used for centuries for their medicinal value as a diuretic and it was only a matter of time before they were added to alcohol. Gin is also flavored with many other botanicals, including but not limited to Coriander, anise seed, angelica root, citrus peels, cardamom, licorice root, orris root, nutmeg, cucumber, rose petal, and cinnamon.
There are many distinctive styles of gin as well, though the vast majority on the market in the U.S. are London Dry Gins.
London Dry Gin:
So called because of its stylistic London roots. Typically column distilled the gin is made in a light and dry style and works well for mixing.
Hailing originally from England's port town of Plymouth, this gin is made in a more heavily-bodied and stronger flavored style than London Dry Gin.
Dutch Gin or Genever:
Genever is distinctive partly because its spirit base is pot-distilled and often contains a percentage of malted grain mash (like whisky). The resulting distillate is lower proof and has a more oily body than its column distilled counterparts. The juniper character comes from an additional distillation with added botanicals, mainly juniper. Genever typically has a slightly sweeter flavor profile but can be dry.
Old Tom Gin:
Old Tom is a version of London Dry style gin that has been slightly sweetened. It is the traditional base spirit for the "Tom Collins" cocktail.
Steinhaeger or German Gin:
Produced by the distillation of fermented Juniper berries. No additional flavor botanicals are permitted.
Varies greatly dependent on botanicals used. Juniper should always be present.
Typically clear though it is not explicitly required to be. Some gins are colored naturally by maceration and allowed to keep the color taken on from their herbal flavoring agents.
Most predominantly juniper, with herbal flavors ranging widely. Some are very pronounced, dry, and almost bitter. Some are subtle, elegant, aromatic, and sweet.
The most typical proof of a bottled gin in the U.S. is 90 proof or 45% alcohol by volume and it is possible to surpass that level of strength, but many releases are 40% abv as well.