Gin is a liquor made from grain (some combination of barley, corn, and/or rye) that has been infused with juniper berries and other botanicals. Juniper berries—small, bluish berries with a piney flavor and aroma—are the predominant flavoring for all gin. Other botanicals that are used to flavor gin include coriander seed, anise, caraway seed, cinnamon, cardamom, citrus peel, ginger, and licorice. The combination of botanicals—along with the production method—is what distinguishes one brand of gin from another.
Re-distilling the grain alcohol with the flavorings is considered the finest method of producing gin; it is also the original method of production, and the most expensive. The cheapest mass-produced gins are made using the cold compound process, by which the extracted flavors are simply added to the alcohol, a practice that is not as highly regarded as distilling.
Dry gin—made primarily from corn, with a small amount of malted barley and other grains—is by far the most common type of gin. As its name indicates, dry gin is not sweet. It is usually aromatic and light in flavor and body. Dry gin originated in England, and most dry gins are labeled "English Dry Gin," "London Dry Gin," or "London Extra Dry Gin"; the names refer to the gin's style, not where it was produced. Plymouth gin is a type of gin that can legally only be produced in Plymouth, England, and is fuller-bodied, smoother, and stronger in flavor than the London style of dry gin. Dry gin is typically mixed in cocktails, most notably gin and tonics or classic Martinis.
You may also see Dutch gin, which is typically made from equal parts malted barley, corn, and rye, resulting in a stronger malt flavor and fuller body than dry gin. Because of its fuller flavor and body, Dutch gin (also called Hollands or Genever gin) is usually drunk cold and straight, not used as a mixer.