Arabica beans and robusta beans are two different species of coffee grown commercially for consumption as coffee. The general differences are those of taste, the conditions under which the two species grow and economic differences.
Taste: Arabicas have a wider taste range, between varieties. They range in taste from sweet-soft to sharp-tangy. Their unroasted smell is sometimes likened to blueberries. Their roasted smell is perfumey with fruity notes and sugary tones.
Robustas taste range is neutral to harsh and they are often described as tasting grain-like, oatmeally. Burnt tires is the description that I personally find most accurate. Their unroasted smell is often described as raw-peanutty. There are high quality robustas on the market but they are rare and reserved exclusively for the best robusta containing espressos.
Production Conditions: Arabicas are delicate, they require cool subtropical climates, lots of moisture, rich soil, shade and sun. They are subject to attack from various pests, and are extremely vulnerable to cold and bad handling. Arabicas also must be grown at a higher elevation of 600 to 2000 meters.
Robustas are hardier plants, capable of growing well at low altitudes of 200 to 800 meters, they are also less subject to problems related to pests and rough handling. They yield more pounds of finished goods per acre at a lower cost of production.
Economics: Customs and trade, supply and demand over the course of the last 150 years has determined the relative values of arabica vs. robusta beans. Generally speaking, the best coffees are all arabicas and the highest quality blends are pure arabica blends. They are also the priciest.
In the U.S. you will generally find arabicas in the coffee store and specialty food shop, and robustas in the supermarket cans. Jars of instant are almost exclusively robusta.
In Italy, home of espresso, the very highest quality brands are pure arabica, and like here, the popular-priced goods are blended with robusta beans. Because "Imported from Italy" can make an ordinary supermarket quality Italian espresso a "gourmet" coffee in the U.S., you will find robustas in some Italian brands offered for sale in the United States.
The coffee you like is a very personal thing. You may find that you really prefer the all-arabica blends, or you may feel comfortable with something less, just because you like it. That's OK. The American marketplace, thanks to the Specialty Coffee movement here, is now rich enough in roast types, species, varieties, blends, brews, grinds, and price points to have something for every taste and pocketbook.
It should be noted that a low quality arabica bean cupped next to a high quality robusta will probably be the inferior bean. So, don't get too caught up in the arabica versus robusta argument. Many great espresso blends use robusta for it's strength and crema.
I should also mention that Arabica does not equal quality. Over seventy percent of the coffee grown throughout the world is arabica. Much of it is garbage so do not assume that just because you are buying arabica you are getting a quality coffee.
One other side note that must be mentioned is that Robusta has approximately twice as much caffeine as Arabica. This may be an issue for some people when choosing their coffee.