Robusta bönan är just robust. Tål mer, och därför lite billigare än Arabica. Robusta brukar man dryga ut Arabica kaffe med i billigare sorter.
Arabica beans grow best at altitudes over 3,000 feet. This species produces superior quality coffees, which possess the greatest flavor and aromatic characteristics. They typically contain half the caffeine of the robusta beans. Arabica production represents 80% of the world's coffee trade, however, only 10% of this meets speciality coffee standards.
Robusta beans are usually grown at lower elevations. Robusta trees are easier to grow, produce higher yields, and are more disease resistant than the arabica species. Robusta beans usually possess a woody, astringent flavor. They are used when a lower price or additional caffeine is desired. A small percentage is typically added to many Italian espresso blends for the additional crema and complexity they contribute.
http://www.allaboutcoffee.org