Black means nothing added. Straight coffee.
Cream, milk, or half-and-half (half milk, half cream) are common additions.
Sugar, or a sugar substitute (Splenda, Equal, etc.), is another common addition. Honey is a sweetener used more rarely instead of sugar or an artificial sweetener.
Some people add cinnamon to their coffee.
To more drastically change the flavor of coffee, there are also flavored (french vanilla, hazelnut, chocolate raspberry, etc.) creamers (Coffee-Mate.com) or syrups you can add.
And finally, you can make coffee an alcoholic beverage by adding something like vodka, tequila, gin, whiskey, chocolate/coffee liqueur or whatever concoction you want to experiment to it.