The two most important (and well-known) coffee plants are: Coffea robusta and Coffea Arabica.
Arabica and Robusta
Robusta and quality
As the requirements regarding growing and harvesting robusta beans are lower than for arabica, the robusta variety the "cheaper" bean and therefore used in lower quality roasted coffees. Unfortunately, this has led to a bad reputation in terms of quality of the bean itself.
This reputation is unjustified - just as there are Arabica beans of lower quality, Robusta beans of very high quality can convince the coffee experts.
Especially Italian espresso blends with a significant portion of (high quality) Robusta are a good example for this misconception.