Gin is a spirit flavored with juniper berries. Distilled gin is made by re-distilling white grain spirit which has been flavored with juniper berries. Compound gin is made by flavoring neutral grain spirit with juniper berries without re-distilling and can be considered a flavored vodka.
The most common style of gin, typically used for mixed drinks, is London dry gin. London dry gin is made by taking a neutral grain spirit (usually produced in a column still) and re-distilling after the botanicals are added. In addition to juniper, it is usually made with a small amount of citrus botanicals like lemon and bitter orange peel. Other botanicals that may be used include anise, angelica root and seed, orris root, licorice root, cinnamon, coriander, and cassia bark.
Other types of distilled gin include jonge- and oude- Jenever or Genever (young and old Dutch gin), Plymouth gin, and Old Tom gin. Compound gin is gin where the juniper flavoring is added to the neutral spirit and there is no re-distillation. Sloe gin is a common ready-sweetened form of gin that is traditionally made by infusing sloes (the fruit of the blackthorn) in gin. Similar infusions are possible with other fruits.
A well-made gin will be relatively dry compared to other spirits. Gin is often mixed in cocktails with sweeter ingredients like tonic water or vermouth to balance this dryness.