Bitters.

So this Italian site divides them into five...

I'll quote:

Infine, in base alla natura chimica dei principi attivi contenuti nelle droghe eupeptiche possiamo suddividerle in:
- amari puri se contengono solo principi amari (Genziana, Quassia, Centaurea, Trifoglio fibrino);
- amari alcaloidei se contengono alcaloidi di sapore amaro (China, Noce vomica, Fava di St. Ignazio);
- amari aromatici se contengono principi amari e oli essenziali (Angelica, Arancio amaro, Assenzio, Calamo aromatico, Luppolo, Ruta, Condurango, Cascarilla);
- amari mucillaginosi se contengono principi amari e forti quantità di mucillagini (Colombo, Lichene islandico);
- amari salini se contengono principi amari a cui si associano elevate quantità di sali (Cardo benedetto, Cicoria, Tarassaco).

I've always divided them into three:

• Amara tonica: simple bitters. Dandelion, chicory, the gentians, hops, bogbean (Menyanthes) etc.
• Amara aromatica: aromatic bitters, these contain essential oils in addition to being bitter. The mugworts, angelica, calamus, yarrow, juniper berries, etc.
• Amara acida: acid bitters; the bitter taste is almost completely buried in an acid or burning taste. Grapefruit, ginger, black pepper, etc.

Comparing the lists:

• pure bitters: "tonic bitters" in my list, and I include dandelion, chicory, and hops. Aye, quassia is extremely bitter, as is centaury (which to me is just yet another gentian). And yes, bogbean is bitter, too.
• alkaloidal bitters: the plants they list (cinchona, nux vomica, ignatius beans) are exotics and some of them are rather toxic; I don't use any of them.
• aromatic bitters: yep, those are aromatic bitters alright (angelica, orange peel, the artemisias, calamus, hops, rue, condurango, cascarilla), except that I think hops is a simple bitter, no aromatics about it; and I haven't even tasted (let alone used) condurango or cascarilla, as they are exotics.
• mucilaginous bitters: actually, I put cetraria into the simple bitters as well, and I haven't used or tasted calumba.
• salty bitters: that's just more simple bitters to me, dunno what sort of salts you'd find in dandelion, chicory and blessed thistle.

They don't list acid bitters at all, although a combination of acid and aromatic bitters is the worst in terms of taste, and the best in terms of efficacy.

... so if you want to give bitters to somebody you don't like, give'em mugwort vinegar. It'll taste ghastly and work a treat. Not that I ever have, I'm much too sweet and gentle a person for that. Honest!

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Related entry: Swedish bitters - Bitters again