Bitters are spirits that are intensely flavored with aromatics, roots and herbal extracts. They get their name from the fact that many of the ingredients commonly used in bitters have something of a bitter flavor, such as orange peel, gentian and quinine. Bitters were originally used for medicinal purposes and many of the elements that flavor them have medical benefits. Over time, they made their way into cocktails that were apéritifs and digestifs, where they could help stimulate the appetite or settle the stomach.
Today, bitters are just used for flavoring and not for any real medicinal purpose. There are a few well known brands, but with the craft cocktail movement there are lots of small producers that are putting out custom bitters with unique flavors that you definitely wouldn’t have seen in the early 1800s when bitters first came into use.