Varieties of coffee beans
There are some great answers here but I have seen that besides the main branches of the coffee family tree there's plenty more to talk about in regards to this fascinating subject. I'll try to avoid digging into ethnobotanist details, or generalizations since a coffee bean's taste depends on as much factors as a wine grape does (eg. sunshine, rainfall, day and night temperature, process method, roast type, et al). But I would like to add value to the discussion by digging further into the 3 types (species) and cultivars mentioned by Mr. Baskerville:
**Arabica branch** -·- The most notorious beans from this species are the Gesha (aka Geisha) beans, known for its cup sweetness, exquisite acidity, and balance. It's famous for fetching astronomical prices during coffee auctions . From my personal perspective there are some other beans like the Java cultivar, the Barnuk Sudan, and the Ennarea that are worth the shot.
**Robusta branch** -·- Ruiru 11, Timor Hybrid as well as Castillo and Colombia varieties are the most common beans from this species. White Tale Coffee (disclaimer: I'm the founder here) has some delicious, with an outstanding aftertaste as its most notorious feature. Disease resistance and high productivity are the biggest advantages of these beans, making it a good choice for small landowners. In my bucket list there's a variety (Oro Azteca) I haven't had the opportunity to try. Certain farms in Brazil are doing wonderful things with Robusta, which I had the opportunity to try during 2015's SCAA Event.
**Liberica branch** -·- Not a very common one, I've run only into a couple of these beans during my short career in the coffee business. I'd love to have exposure to these beans and get to know them better - especially varieties from Burundi like S288. Unfortunately I haven't had the opportunity to do so.
**Cultivars from the Yemen Accession** -·- Well, this is one of the cornerstones of the famous 3rd wave of coffee and there's plenty of material to indulge in. Here's a shortlist of what you can find in specialized coffee websites and shops, all of them stemming from Typica and Boubon varieties:
- SL-34 and SL-34
- Laurina (known for its low caffeine content)
- Orange, Pink, Red, and Yellow Bourbon
- Blue Mountain
Each one of them has it respective nuances and characteristics. Above all, we have to take into account that coffee is an agricultural product and the atmospheric conditions it grows in affect its taste significantly. Hence the world of flavor notes the coffee industry has been working to understand it better. See for more information.