When talking about soft drink, a misunderstanding may arise between representatives of different parts of the world. In New Hampshire people call all carbonated drinks Tonic, in Denver - Tonic water, in Boston you won't find Tonic water in supermarkets - it is called Quinine Water there. In the USA Tonic water is a specific drink that Americans use for Gin Tonic, not a generic term.
Sometimes Tonic is a synonym of Tonic Water, sometimes it's not. But the cocktail recipes usually include Tonic Water. So, when we talk about Tonic we mean Tonic Water that is indeed the one we mix with gin.
Tonic water contains quinine (due to quinine Tonic water glows under a black light), which was used to prevent malaria. That's why in the 19th century it was extremely bitter, gin was added to make it more palatable. Although tonic water is less used for medical purposes today, the gin and tonic remains a popular drink. Note that tonic water available today contains less quinine and is consequently less bitter (and usually sweetened). You can find more information on the previous usage of Tonic water on the internet if you are interested.
The best-known brands of Tonic Water are Canada Dry, Cantrell, Schweppes, and Nordic Mist of Coca-Cola Company. Some manufacturers also produce diet tonic water respectively. In Europe you can find Tonic water with lemon or lime flavor added, which are known as bitter lemon or bitter lime. Such soft drinks are more popular in Europe than in the United States.
Don't mix up Tonic Water with Club Soda. They are both carbonated soft drinks, but Club soda is not flavored with quinine and Tonic water is.
Tonic water is often used as a mixer for alcoholic spirits, especially gin (the mixture commonly known as a gin and tonic).The Gin and Tonic is a cocktail made with gin and tonic water, and served over ice. The ratio one to one is traditionally used, but proportions may vary up to your taste. Make it strong at the traditional ratio or weak, typically at the ratio of 1 part gin to 3 parts tonic, or 2 part gin to 3 parts tonic. Stir well, garnish with lime wedge. You can find various recipes of the drink and it is up to you which one to try.
Because of Gin and Tonic's refreshing nature this cocktail is most popular during the warmer months. A range of quality non alcoholic wines may be ideal as cocktail mixers that can be mixed with tonic water.
Surely the Internet is the best source of information. On related sites you may find 31 the most popular cocktails with tonic water, including Federal Law, Olle Goop, Cherries from Heaven, Chartreuse and Tonic, Bahamas Highball and others, and even the baby milk cocktail.
There are also some refreshing cocktail recipes with Schweppes Tonic Water.
You may as well try stirred Boggy Creek Bloody Mary with vodka and Tomato tonic. Tomato Tonic is a delicious combination of tomatoes, onions, vinegar, and some secret ingredients (including a slight hit of serrano) that go perfectly with Tito's Handmade Vodka.