Difference between Arabica and Robusta

Arabica (coffea arabica)
Coffea Arabica is believed to be the first species of coffee to be cultivated, being grown in southwest Arabia for well over 1,000 years. arabica coffee beans are grown at a high altitude and typically found in the subtropical/equatorial regions of Costa Rica, Colombia, Ethiopia and Kenya. Arabica coffee is often susceptible to attack by pests and diseases (especially typica, being lower in caffeine), therefore resistance is a major goal of plant breeding programmes. Arabica coffee is used predominately in blended coffee because of its properties of sweetness, acidity and fruit/citrus driven flavours. It generally has a lighter 'body', a term that combines the elements of total dissolved solids and subjective mouthfeel.
Arabica coffee is characterized by its sweetness and acidity, its fruity and zesty flavours and intense aroma. It has a light to medium body and a soft mouthfeel. It contains on average about half of the caffeine of Robusta.

Robusta (coffea canephora)

Robusta coffee is characterized by its woody and earthy flavours. It has lower acidity and a heavier mouthfeel. In addition to the fact that Robusta is cheaper in comparison with Arabica, it offers several very important advantages of its own. As we have already mentioned Robusta beans contain bigger amount of caffeine.  It is traditionally blended with Arabica beans to achieve excellent body in an espresso.
The Robusta trees are high bearing and disease resistant. The fruits are rounded and take up to 11 months to mature (rather than 5 years as with Arabica); the seeds are oval in shape and smaller than those of coffea Arabica. Robusta is easier to grow, cultivate and is more robust (pun is obvious) than Arabica. Robusta coffee generally has less sweetness and a woody, earthy character with a heavy body. There are even those that claim there are Robusta's that are better than Arabicas. On the other hand Robusta moved to Asian market where it managed to become even more popular than Arabica due to its low price and good coffee qualities.