Arabica (Coffea arabica), named for the Arabs, is the better and more expensive of the two. These trees grow in semitropical climates near the equator, both in the western and eastern hemispheres, at high altitudes. Because ripe Arabica cherries (unroasted beans) fall to the ground and spoil, they must be carefully monitored and picked at intervals, which increases production costs.
Robusta trees (Coffea canephora), which are grown exclusively in the eastern hemisphere, also thrive in equatorial climates, but at low altitudes. Their cherries require less care since they remain on the tree after they ripen. Robusta beans have twice the caffeine of Arabica, but less flavor. Some supermarkets carry Arabica, but most of their brands are robusta. Coffee shops generally use Arabica beans, but because their brews are so strong, I don't enjoy them. I had always been satisfied with the various Maxwell House roasts and blends, until I bought my first bag of Arabica beans, ground them, brewed and drank the elixir.
To describe the taste of Arabica is difficult without using trite words like smooth and mellow. It has a round taste that is both rich and delicate, with good acidity. This does not refer to an actual degree of acidity, but to the sharp and pleasing taste that is neither sour nor sweet. Although most robusta coffee is of a lower grade and inferior to Arabica, there is a premium crop that is the top of the line for robusta beans. Premium robusta is primarily used in specialty espresso blends, and is never found in canned coffee. Though it only constitutes 5-15% of the blend, it is used because these beans add body to the taste and make a nice creama in the shot of espresso. This additional body distinguishes the blend in a cappuccino or latte. Premium robusta should only be used for espresso and not other brewing methods.