Originating as a medical elixir of sorts, Gin was originally called genever, which is the Dutch word for juniper. When the drink became popular in England, its name was shortened to gin.
Gin is made by distilling fermented grain (usually corn) and and very slowly passing the spirit through juniper berries and sometimes other herbs and botanicals to give it its distinct flavor. Just like with other alcoholic beverages, not all gins are created equal. You have your quality brands, such as Tanqueray, Beefeater and Bombay, and you have also have no-name rotgut poison. Good gin is supposed to taste like juniper berries, whereas bad gin tastes like disinfectant.
Chances are, if you simply say to the bartender that you would like a gin and tonic, a Tom Collins, a gimlet, a Martini or a negroni, the bartender might use cheap, Brand X well gin, which will only inevitably spoil the cocktail. Luckily, there are bartenders that will ask you which brand of gin you prefer. Personally, I feel as though such a practice should be standard. However, just in case, make sure you name your brand and always go for the good stuff.