The botanical varieties of Coffea arabica — like grape varieties for wine, or apple varieties for cider. Two coffees from the same farm, processed the same way, will taste different if they're different varietals. Specialty bags often print the variety alongside origin and processing.
The big arabica varietals
Typica. The historical baseline. Mild, classic balance. Most "old world" South American coffees descend from typica.
Bourbon. A natural mutation of typica with higher cup quality and slightly lower yield. Sweet, balanced, complex. The benchmark for excellence in many origins. Variants: yellow bourbon, red bourbon, pink bourbon.
Caturra. A dwarf mutation of bourbon. Compact plants, easier to harvest, retains most bourbon character. Backbone of Colombian and Central American specialty.
Geisha (Gesha). The famous one. Originally from Ethiopia, made famous by Hacienda La Esmeralda in Panama in 2004. Floral, jasmine, bergamot, sometimes papaya. Wins competitions, costs accordingly. The single varietal most likely to make a non-coffee drinker say "wait, that's coffee?"
SL28 / SL34. Kenyan workhorses developed by Scott Laboratories in the 1930s. The reason Kenya tastes like blackcurrant — phosphoric acidity unmatched elsewhere.
Pacamara, Maragogype. Large-bean varietals from El Salvador and Brazil respectively. Distinctive cup characters; pacamara especially is showy and intense.
Why varietals matter
If you've tasted a Geisha and an SL28 from similar altitudes and processing, you've experienced the variability that origin and farm alone don't explain. The variety is doing real work. Reading varietal info on a bag — and remembering which ones you liked — is one of the fastest paths to understanding your own taste preferences in coffee.