There are hundreds of varieties, but only about 10 are commercially grown. The two major species are: Coffea robusta and Coffea arabica. Both have many varieties, and there are hybrids deriving from both.
It is enough to know that they are quite different for very good reason. Their growing habitats are uniquely different.
Simply put, Coffea robusta is grown at lower altitudes where sunshine, moisture, rich soil, and heat produce an excellent seed germinating environment. The tree, realizing this, does not need to produce a seed with a lot of energy food needed for germination in harsher growing conditions, so it preserves that energy by producing a minimalist seed with less food (starch) and more waste products (caffeine). The caffeine is a natural insect repellent, and at lower altitudes, there are more insects than at the higher altitudes where Coffea arabica reigns.
What you need to know is that Robustas are less sweet, less nuanced, and mostly found as filler in canned coffees and as the main coffee in instants.
Coffea arabica is the Ferrari of coffees with varieties such as Typica and Bourbon. This tree needs to put lots of energy food into its seeds because it operates in a difficult environment where the air is cooler because of altitude (3000-6000 feet), the rain is very seasonal, the soil is thin, and the sun is shaded so the soil may be cooler. The tree, recognizing its seed may need additional time to germinate and get its leaves pointing at the sun, produces multiple kinds of starches and oils from which the seed can draw its initial power. It is these energy foods that create the great coffee flavors when they are roasted by the artisan roaster.
A side benefit is lower caffeine content. The photosynthetic energy went into food production instead of waste production, of which caffeine is but one of 1600 compounds found in coffee.
It is the Arabica varieties and their hybrids that comprise the specialty coffees we love.