There are more than 120 species of the coffee tree, but 80% of the coffee produced each year comes from the Coffea arabica plant. The second most common coffee produced is the Coffea canephora, better known as Robusta.
Robusta is, as you might have guessed, a more robust plant with higher resistance to heat and disease. It has twice the caffeine content of Arabica; in fact, this caffeine concentration acts as a natural pesticide.
Coffee growers were thrilled to discover this alternative to Arabica in the late 19th century; for all its strengths though, Robusta has a downside - its taste is harsher and more bitter.