It’s the age old question by the caffeine addict – which is the better bean? The classier, subtler, harder to grow arabica, or the hardy, beefy, robust robusta bean?
Arabica (coffea arabica), named by the Arabs, is the 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 lower altitudes. Their cherries require less care since they remain on the tree after they ripen. Robusta beans have twice the caffeine of arabica (which is why the Italians began using it for espresso), but less flavour. Some supermarkets carry arabica, but most of their brands are robusta. 
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.
In fairness, 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 speciality espresso blends, and is never found in instant coffee. It is used because these beans add body to the taste and produce a nice crema on the shot of espresso. This additional body distinguishes the blend in a cappuccino or latte and premium robusta should only be used for espresso and not other brewing methods. In general, an espresso shot benefits from the inclusion of robusta with the arabica content, the degree of which is a matter for debate - that's both the optimum robusta/arabica ratio aswell as how much of an improvement this leads to. These factors are of high importance when selecting a brand of ESE pod (Easy Serving Espresso 44mm coffee pod). Here, the spectrum of arabica:robusta content varies considerably from 100% Arabica to High Robusta (90%) pods.  
An arabica bean is flatter and more elongated; in addition, and the furrow on its flat surface is elongated. It is relatively deep green in color before roasting, sometimes with a bluish tinge.
The robusta bean is more convex and roundish. The bean’s furrow is straight, and it is pale green with grey or brownish tinges.
The arabica, with twice as many chromosomes (44) as the robusta, has great complexity to it, which makes it a great home choice, but the robusta is really the bean that has made low-cost coffee drinking possible. If you can take its slightly more bitter taste (and many prefer it), it’s a great inexpensive option.