There are basically 2 types of Green Coffee Beans - Robusta Beans are grown at low altitudes, are more tolerant to climate change and threats to the coffee tree.  These beans are higher in caffeine but are generally bitter when roasted.  These beans are less per pound and makeup a lot of your instant and supermarket varieties. 
The 2nd. type are Arabica Beans - these beans are grown at higher altitudes, are less prevalent but open up a new world to wonderful tastes upon roast.  Roasting Expectations, a small batch Specialty Roaster only sources these more expensive beans for your enjoyment,