A liqueur is a flavored spirit that contains added sugar. Some of the flavors include herbs, nuts, fruits, flowers and spices. These liqueurs have a level of sweetness to them. A majority of the liqueurs were originally used for medicinal purposes like ailing stomachs and digestion. These liqueurs have a lower alcohol content than regular spirits and are used in many cocktails. Some of these liqueurs fall under different categories.
Aperitif is traditionally served before a meaLil and meant to awaken your appetite. This practice was popular in the 19th century in many European countries.
Digestif is the opposite of Aperitif and is served after a meal.  It is said to help with digestion. Some refer to them as bitters which are herbs added and known to prevent the formation of gases in the stomach. Digestif have more alcohol than aperitifs. Some wines are substituted for Digestif like sherry, ports or Madeira.
Cordials can contain extract of fruit flavorings. These are sweet, syrupy and aren’t usually aged for long. They contain added sugar. Sometimes Cordial can mean a non alcoholic mixer. They were originally intended to be medicines and were consumed to renew the spirit and fight disease.
Schnapps are fruit flavored liqueurs that are usually added to cocktails. True German schnapps do not have added sugar although in the United States they do add sugars. The name is German and translates to the word swallow.
These are some of the terms that fall under the category of Liqueur although this is debatable. At one time these terms had strict definitions but through time they have become interchangeable terms. Many other wines can sometimes substitute for an aperitif or digestif.