Bartenders use a wide variety of ingredients when preparing drinks for their customers. Mixed drinks can be quite complicated to prepare and may use many different components in a single drink.
Most bar drinks contain some type of alcohol. Fermented beverages, such as beer or wine, are served at a wide variety of bars. In the United States, beer and wine only alcoholic beverage permits are much easier to obtain than permits allowing distilled beverages like spirits and liqueurs.
In addition to beer and wine, other common fermented beverages include cider (called hard cider in the U.S.), malt beverages (usually referring to clear malt-based beverages with fruity or other "fun" flavors that appeal to younger drinkers), mead, sake (commonly served in Asia and places that serve Asian food), and vermouth (a type of wine used in many cocktails).
Common spirits include absinthe, brandy, gin, rum, tequila, vodka, and a variety of whiskies. Less common and regional spirits are available, too. Liqueurs are sweetened alcohols with flavors like chocolate, coffee, melons, oranges, other fruit and berries. Nuts and herbs are other popular flavors.
Of course, some people prefer non-alcoholic beverages, and most mixed drinks can be made without alcohol (so called "virgin" drinks). Most bars offer drink staples like sodas and coffee, and larger bars may offer iced or hot tea and bottled water. Many will provide juice or milk upon request. Bars serving younger crowds also may serve energy drinks upon request or mix them into other drinks for a caffeine "kick".
Mixers are an important part of most mixed drinks, whether alcoholic or not. As mentioned, caffeinated beverages like coffee, tea, and energy drinks are sometimes added to drinks to counteract the suppressant effect of alcohol.
Syrups, like simple syrup, add sweetness, and some, like grenadine, also add pleasant color to the drinks. Other syrups, like sour mix and sweetened lime juice add both a sweet and a sour taste to drinks. Bitters impart a bitter taste to drinks. Bitters are an important flavor in many cocktails, and are a required ingredient in true cocktails according to purists.
Other mixers include a variety of carbonated mixers (including carbonated and tonic waters, and also citrus, cola, ginger, or sarsaparilla flavored sodas), juices (including tomato and vegetable juices, tropical-flavored juices, and other fruit and berry juices), dairy products (like milk, half and half, heavy cream, or ice cream), and even hot or savory sauces.
The final ingredient is the garnish. Many drinks include inedible garnishes like straws or little paper umbrellas or plastic animals. Other garnishes, like citrus peel twists, are not really edible, but the same fruit can be quartered or sliced, and then it becomes an edible part of the drink experience. Other edible garnishes include tropical fruits like pineapple and coconut, cherries, and vegetables like carrot or celery sticks, green olives, or cocktail onions. Even spices and seasonings like cinnamon, cocoa powder, mint sprigs and leaves, salt, and sugar can be used to enhance a drink.
All of these ingredients are served to the customer in some type of drinkware.