Not all coffee beans are created equal, a fact bemoaned by the great divergence in quality across the coffee industry. Most coffee sold in supermarkets contain a blend of Arabica and robusta coffee beans while most coffee shops sell only Arabica beans. Arabica beans are usually flavor rich, while robusta beans have more caffeine, less flavor and are cheaper to produce. The exception to this rule is that some very good espresso coffees will have small amounts of the highest quality robusta beans on the market. This is not a guarantee that a local coffee shop will have any better coffee than the local grocery store.
When you buy coffee beans, whether from a coffee shop or in a supermarket, you want to get 100% Arabica, except for espresso blends, which may be a combination of both. Whether good quality robusta improves the flavor of espresso is up for debate but in all likelihood this is a debate that will linger for quite some time. The robusta does contribute heavily to the caffeine content of an espresso and is considered vital to a proper blend.