According to Wikipedia,
“Gin is a spirit which derives its predominant flavor from juniper berries (Juniperus communis)”.
There are many styles of gin (almost endless), so let us slim down the field to three styles: English/London Dry style, American style, and Dutch/Holland Genever style.
English/London Dry-style Gin
English/London Dry-style gin is made using neutral grain spirits. The juniper and other botanicals (such as lemon, anise, almond, cinnamon, saffron, frankincense, coriander, cucumber, and cassia bark to name a very few) are introduced during re-distillation in a “gin basket”. This type of gin cannot contain added sweetening in order to retain its dry quality and is usually a high proof (from 80 to 95 proof).
American-style gin is also made from neutral grain spirits, but the juniper and botanicals are added after distillation as an infusion. This type of gin has many names, such as compound gin, bathtub gin, and cowboy gin. American-style gins are also high-proof (from 80 to 95 proof). This is the style of Crater Lake Gin at 95 proof.
Dutch/Holland-style Genever is the earliest style of gin. It is distilled from barley malt and other grains, then the juniper and botanicals are added. Sometimes it is aged in wood barrels, which gives it a resemblance to whiskey. Genever usually has a lower proof (from 60 to 80 proof).