About Bitters
There are two basic types of bitters in the spirit world. Both originated as early (created by , or at least first documented by – medieval monasteries medicine as digestifs made from a combination of botanicals and a alcohol base. They have since fallen into 2 more or less distinct types the term Bitters itself is a short hand for both:
1. BITTER TINCTURES
A type that is a concentrated mixture of plant essences/flavorings used in small amounts in the blending of drinks. Alcohol is used for the base in the majority of these due to the extraction/solubility properties of alcohol, but some may use glycol or water.
Examples of type 1 are:  Angostura Bitters, Regans Orange Bitters, Peychauds Bitters, or similar.

2. BITTER TONICS

A type of distilled spirit that contains many flavorings that is used in much larger quantities than the above in drinks.
Examples of type 2 are: Campari, Aperol, Fernet Branca, or Killeptsch.

Among the type 2 Bitter Tonics you will see the terms Amaro and Fernets among the Italian Bitter Tonics,

The distinguishing characteristic for an Amaro is a fruit/citrus forward Bitter Tonic i.e.Campari, Aperol etc.,),  Whereas a Fernet is an herbal/root forward Bitter Tonic ( Example Fernet Branca or Fernet Menta )
A bitter may sometimes also be classified in the liqueur family as well depending on a number of factors, including marketing.