If you are looking for the heart of the cocktail for someone who may not care for strong alcohol, the liqueur is it.
A liqueur usually has a lower percentage of alcohol, compared to a liquor. It often brings a fair amount of sugar to the game which makes the alcohol seem to have less “burn”. And let’s not forget the flavoring which is the whole point. It can be a natural or artificial flavor.
In the cocktail world, probably the most well know liqueur is triple sec. If you’ve had a margarita, you’ve had triple sec. There is a large price differential from the bottom shelf to the top. I know there are people out there who can tell the subtle differences between a DeKuyper triple sec and Cointreau or Gran Marnier, but once buried in a cocktail with six other ingredients, I’d hazard that it doesn’t make a huge difference for most people.
An orange liqueur is generally produced “from the dried peels of bitter and sweet orange” (Wikipedia). This adds a hint of orange flavor, aroma, and sweetness to cocktails.
Limoncello is typically made from the zest of tart lemons that have little bitterness. Lemon zest, or peels without the pith, is steeped in a base spirit until the oil is released. The resulting yellow liquid is then mixed with simple syrup (Wikipedia link).
Non-citrus Fruit Liqueurs
Fruit Liqueurs are made from a neutral grain spirit, real or artificial fruit flavors, and sugar.
Almond liqueurs are produced from base of apricot pits, peach pits, or almonds.
Floral liqueurs are made from flowers that are steeped in a base spirit and typically with sugar added
Whiskey or Whiskey Flavoring are sometimes used in the productions of liqueurs.
Coffee, Chocolate, Mint, Butterscotch and cream flavors are used to produce very dessert-like liqueurs