There are a lot of coffee species. Two of them are commercially serious, and one of those two is on almost every bag you’ve bought.
Arabica
Roughly three-quarters of the world’s coffee. Sweeter, more acidic in the pleasant sense, and far more varied — an arabica from Ethiopia and an arabica from Colombia can taste like they have nothing to do with each other.
It’s also fussy. It wants altitude, shade, a narrow temperature band, and it catches diseases readily. Which is exactly why it costs more, and why the coffee industry spends so much time worrying about the weather.
Robusta
Most of the rest. It grows lower, hotter and faster, shrugs off disease, and yields far more per tree. It has close to twice the caffeine, which is part of why it tastes harsher — caffeine is bitter, and the plant makes it as a pesticide rather than as a favor to you.
Robusta got its reputation honestly. For decades it was the cheap filler in supermarket tins, grown for volume with no attention paid, and it tasted like it. “100% arabica” became a marketing line because the alternative really was worse.
The part that’s changed
Robusta grown carefully, at altitude, picked ripe and processed well is a genuinely good drink. There isn’t much of it, and you’ll pay for it, but the flat assumption that robusta means bad is about fifteen years out of date.
It’s also the backbone of a lot of Italian espresso, where a little robusta in the blend gives the crema its body. If you’ve had an espresso in Italy and wondered why it was so thick, that may well be why.
There’s a longer argument for the defense on its card in The Lineup, where we make the case at more length than is strictly polite.
So what should you buy?
Arabica, almost certainly, because that’s what specialty coffee is and it’s where all the interesting stuff is happening. But “100% arabica” on a bag is a floor, not a recommendation — it rules out the worst option and tells you nothing else. Plenty of dull coffee is 100% arabica.
What tells you more: a roast date, the name of a specific place, and somebody willing to say who grew it. That’s the argument in what “single origin” means.