Ah, the Gin and Tonic. The magical drink that glows, cures malaria, and makes women beautiful.
The Gin and Tonic was originally made in India by the British East India Trading Company in the 18th century, to treat their soldiers. That's right, this drink was originally a medicine. This is probably one of the earlier instances of abusing meds.
The tonic is what was medicinal. It contains quinine, which prevents malaria. The tonic water used at the time had much higher concentrations, and as such, was unspeakably bitter. So people would conceal the taste by mixing it with gin.
In the preparation of the Gin and Tonic, you need two things. For the really slow readers, those two things are Gin and Tonic Water.
Take a highball glass, and fill it with ice. Then add 2 ounces of gin, fill the rest with tonic water, and stir briskly. a wedge of lime is the traditional garnish, but you can throw just about anything in there. Sit back, sip your drink, and hum God Save the Queen.
With a highball glass, you'll get a 1 to 2 concentration of gin to tonic water. One of the beautiful things about this drink is the ease with which the proportions can be adjusted. I've had Gin and Tonics that ranged from a glass if gin with a hint of tonic water, to a splash of gin to cut the tonic. If you like a more concentrated drink, but don't want a highball sized concentrated drink, you can also adjust the glass you're drinking from. An Old Fashioned Glass works nicely for that.
Another curious feature of tonic water is that it glows under a black light. If you're the type of person who likes your drink at a certain concentration, this is actually a pretty good way to check the concentration. The brighter it glows, the less gin is in it. If your friends are jokers, and they buy you a glass of gin, and try to disguise it as a gin and tonic, you have a way to keep yourself from being caught off guard.
A common variant of the Gin and Tonic is the Vodka and Tonic. I often order this at nightclubs, because low end vodka is much more palatable than low end gin.