Coffee, like most plants, has a ton of diversity within its species. The different varietals of Coffea arabica have all sorts of different traits, some affecting the flavor of the coffee they produce and others making the plants more suitable to different environments. There's thousands of different sub-species and cultivars of coffee - but here's a quick run-down of the ones that crop up most frequently in specialty coffee.
There are so many more varieties out there that have been identified (and even more that haven't) so we're just going to call it here with the basics. But understanding the varietals and health of the species is crucial to fostering a sustainable coffee market going forward, and lots of hardworking folks all around the world are putting effort into research and development of more coffee information and resources. This is just a quick primer for coffee drinkers - but organizations like World Coffee Research are putting together comprehensive resources for producers, roasters, and other coffee professionals. Check it out!