A Liqueur, by legal definition, is a drink that is composed of a base spirit, water and at least 100 grams per Litre of sugar. It can also have cream, milk, fruit or wine added to it as well. Most often liqueurs also have a wide range of colours and flavourings to give them mass appeal and make them unique in the world of spirits.
There are a few different types of Liqueurs, though, which differ largely by the base spirit used. By this we mean if they have used a flavourless, neutral base spirit (not vodka), if they have produced the liqueur through a process akin to gin manufacturing, or if they have used an already produced spirit like a whisky or rum.
These will largely be made by macerating or flavouring a neutral base spirit with a particular fruit flavour. Examples of these are Limoncello, Triple Sec and various Curacaos.
Many famous examples of fruit liqueurs exist which have had fruits macerated in or blended back into an already produced spirit. Examples of these are Cointreau (orange mixed with Triple Sec) and Grand Marnier or Mandarine Napoleon (orange mixed with Cognac brandy).
Herb- or nut-based Liqueurs
These are some of the most intensely flavoured and complex liqueurs of them all. Like the basis of fruit liqueurs where fruits are macerated into a neutral spirit for a period of time, these are made in exactly the same way just with botanicals. Often these liqueurs will have solely local botanicals (herbs, fruits, nuts, spices, vegetables and flowers) to give a really pure and refreshing take on a particular region or country. Some of the world's leading examples are; Chartreuse, Galliano, Kummel, Sambucca, Amaretto, Strega and Benedictine.
There is a sub-group to this category of liqueurs, though, which use already produced spirits as the base. Examples of these are Malibu (coconut with white rum), Tia Maria (coffee with a sugar-cane spirit akin to white rum) and Kahlua (coffee with a pure white rum).
Then there are a form of herb- or nut-liqueurs which solely use whisk(e)y as a base to macerate, or add, flavourings into. Examples of these are; Drambuie (cloves, nutmeg and honey), Glayva (orange, herbs and honey), Loch Fyne (sloe berries and honey), Southern Comfort (various ingredients but largely peaches).
Definitely one of the most popular and best-selling categories of liqueurs around. Everyone who has ever had a look at the liqueur cabinet in the store or at your parent's home will know of, and most likely tried, Baileys. Bailey's is without a doubt the leader in this field. It, along with others in this style, are produced from milk solids, sugar and alcohol. The sugar is mixed with neutral alcohol, then homogenised a couple of times. This mixture is then cooled. Once cooled the milk solids are whisked in along with the flavourings producing a rich, luscious creamy alcohol mixture.
Not to be confused with the above 'cream liqueurs'. These are a particular style of fruit liqueurs which are produced solely with flavour concentrates and have much higher minimum sugar levels required for legal requirements. The two most famous examples are creme de menthe (mint) and creme de cassis (blackcurrant).