There are many different varieties of coffe, but cafea arabica and cafea robusta are two different species grown in my region of Colombia, robusta specifically grown for its increased resistance to disease and pests. But how are these two species differentiated in the quality of the prepared drink?
Robusta is often used to "cut" blended coffee because it is most often cheaper to purchase than Arabica beans. Since it generally has a higher level of caffeine than Arabica, it may also be used to up the "kick" of blended coffee.
Robusta is generally though to have an inferior flavor to Arabica, partially because the extra caffeine imparts some bitterness, but also perhaps because of it's growing conditions. While some very few places out there may strive to produce superior Robusta beans, they are working against a market that generally considers it an inferior product and will not pay as much for it. Therefore less care is often given to Robusta plants and processing since the return on investment is lower.